High power femtosecond laser with adjustable repetition rate

ABSTRACT

Designs and techniques for constructing and operating femtosecond pulse lasers are provided. One example of a laser engine includes an oscillator that generates and outputs a beam of femtosecond seed pulses, a stretcher-compressor that stretches a duration of the seed pulses, and an amplifier that receives the stretched seed pulses, amplifies an amplitude of selected stretched seed pulses to create amplified stretched pulses, and outputs a laser beam of amplified stretched pulses back to the stretcher-compressor that compresses their duration and outputs a laser beam of femtosecond pulses. The amplifier includes a dispersion controller that compensates a dispersion of the amplified stretched pulses, making the repetition rate of the laser adjustable between procedures or according to the speed of scanning. The laser engine can be compact with a total optical path of less than 500 meters, and have a low number of optical elements, e.g. less than 50.

TECHNICAL FIELD

This patent document relates to femtosecond lasers including adjustablerepetition rate high power femtosecond lasers.

BACKGROUND

In many of today's ever more challenging laser applications there is acontinued quest for shorter pulses which carry high energies per pulse.These features promise better control and greater operating speed forlaser applications. A notable step in the evolution of the field was theappearance and maturation of laser systems outputting femtosecond laserpulses. These femtosecond lasers can be used for a wide variety ofapplications, including several different types of ophthalmic surgeries,where these ultra-short pulses can offer well-controlled tissuemodification.

SUMMARY

Designs and techniques for constructing and operating femtosecond pulselasers are provided in this document, including examples andimplementations of laser systems with chirped pulse amplification, someof which have a low number of optical elements, some have a lowfrequency of malfunctions, others have a suitably small physical extent,yet others can allow the change of the repetition rates withoutsubstantial readjustments of the system, and some have reducedsensitivity for thermal lensing.

For example, some examples of a laser engine include an oscillator thatgenerates and outputs a beam of femtosecond seed pulses, astretcher-compressor that stretches a duration of the seed pulses, andan amplifier that receives the stretched seed pulses from thestretcher-compressor, amplifies an amplitude of selected stretched seedpulses to create amplified stretched pulses, and outputs a laser beam ofamplified stretched pulses, wherein the stretcher-compressor receivesthe laser beam of amplified stretched pulses, compresses a duration ofthe amplified stretched pulses, and outputs a laser beam of femtosecondpulses with a pulse duration of less than 1,000 femtoseconds, and theamplifier includes a dispersion compensator that reduces a dispersion ofthe amplified stretched pulses.

In some examples the oscillator is a diode pumped fiber oscillator andoutputs transform-limited seed pulses.

In some examples, the oscillator generates the beam with aseed-pulse-duration of less than 1,000 femtoseconds.

In some implementations the oscillator outputs the beam with a seedpulse repetition rate in the range of one of 10-100 MHz and 20-50 MHz.

In some implementations the stretcher-compressor includes a chirpedvolume Bragg grating.

In some implementations the stretcher-compressor includes a photothermalrefractive glass.

In some implementations the stretcher-compressor stretches a duration ofthe femtosecond seed pulses by a factor greater than 10.

In some implementations the stretcher-compressor stretches a duration ofthe femtosecond seed pulses to a stretched duration of 1,000-200,000femtoseconds.

In some implementations the laser engine does not contain a tunablestretcher-compressor.

In some implementations the laser engine includes a polarizer and a λ/4plate between the oscillator and the stretcher-compressor that redirectsthe beam of stretched seed pulses toward the amplifier.

In some implementations the laser engine includes a Faraday isolatorthat receives the beam of stretched seed pulses from thestretcher-compressor, outputs the beam of stretched seed pulses towardthe amplifier, receives the laser beam of amplified stretched pulsesfrom the amplifier, outputs the laser beam of amplified stretched pulsestowards a compressor port of the stretcher-compressor, and isolates theoscillator from the laser beam of amplified stretched pulses.

In some implementations the amplifier includes optical elements, and thedispersion compensator introduces a dispersion opposite in sign to adispersion introduced by an optical element of the amplifier.

In some implementations the dispersion introduced by the dispersioncompensator is essentially equal in magnitude and opposite in sign to adispersion introduced within one roundtrip by the optical elements ofthe amplifier other than the dispersion compensator.

In some implementations the dispersion compensator includes at least oneof a chirped mirror, a chirped fiber, a chirped grating, a prism, or achirped transmissive optical element.

In some implementations the amplifier includes a gain material thatamplifies the amplitude of the selected stretched seed pulses, twoend-mirrors that define a resonant cavity, and two folding mirrors thatfold a resonant optical pathway inside the amplifier, wherein at leastone of the two end-mirrors and the two folding mirrors is a chirpedmirror.

In some implementations the chirped mirror introduces a negativedispersion to the amplified stretched pulses.

In some implementations the laser engine is configured to output thelaser beam with a first repetition rate, and subsequently with adifferent second repetition rate with essentially the same setup of alloptical elements of the laser engine.

In some implementations the first repetition rate and the secondrepetition rate fall within one of the ranges of 10 kHz-2 MHz, 50 kHz-1MHz, or 100 kHz-500 kHz.

In some implementations the laser engine can be modified to output thelaser beam with the second repetition rate with essentially the samesetup of all optical elements as with the first repetition rate, whenthe unmodified laser engine utilized different setups of the opticalelements for the first and second repetition rates.

In some implementations the amplifier is configured to have a number ofroundtrips of the amplified stretched pulses in the amplifier changedwhen a repetition rate is changed while keeping an optical setup of theamplifier unchanged.

In some implementations the amplifier has an end-mirror-to-end-mirrorfolded optical pathway of less than 1 meter.

In some implementations the amplifier is a cavity dumped regenerativeamplifier, a chirped pulse amplifier or a Q-switched amplifier.

In some implementations the amplifier includes a switchable polarizer inan optical pathway between end-mirrors that can select stretched pulsesby switching between a polarization-adjusting state in which theswitchable polarizer adjusts a polarization of the amplified stretchedpulses and a polarization-non-adjusting state in which the switchablepolarizer essentially does not adjust the polarization of the amplifiedstretched pulses.

In some implementations the laser engine can include a high voltagepower-switch that controls the switchable polarizer to switch from thepolarization-non-adjusting state to the polarization-adjusting statewith a rise time of less than 5 nanoseconds, 4 nanoseconds or 3nanoseconds.

In some implementations the laser engine changes a first repetition rateof the laser beam of femtosecond pulses to a second repetition ratewithin one of 1-120 seconds, 10-60 seconds and 20-50 seconds.

In some implementations the laser engine changes a first repetition rateof the laser beam of femtosecond pulses to a second repetition ratewithin a changing time in the range of 1 μs-1 s.

In some implementations the amplifier includes at least one focusingmirror and a laser crystal, disposed in close proximity of a focal pointof the focusing mirror.

In some implementations the laser engine is configured so that when arepetition rate of the laser engine is changed from a first value to asecond value, both values in the range of 10 kHz-2 MHz, then theoutputted laser beam's diameter changes by less than one of 10% and 20%,or the outputted laser beam's center moves by less than one of 20% and40% of the beam's diameter.

In some implementations the femtosecond pulses of the laser beam have anenergy in the range of one of 1-100 μJ/pulse, 10-50 μJ/pulse, or 20-30μJ/pulse.

In some implementations the laser engine outputs a laser beam with apower greater than one of 0.1 W, 1 W or 10 W.

In some implementations the laser engine is part of an ophthalmicsurgical system.

In some implementations a method of generating a laser beam with a laserengine includes the steps of: generating a beam of seed pulses withduration less than 1000 femtoseconds with an oscillator; stretching aduration of the seed pulses with a pulse stretcher; amplifying anamplitude of selected stretched seed pulses with an amplifier togenerate amplified stretched pulses; compressing a duration of theamplified stretched pulses to below 1,000 femtoseconds with a pulsecompressor; outputting a laser beam of femtosecond pulses with a firstrepetition rate in the range of 10 kHz-2 MHz and with a pulse durationless than 1,000 femtoseconds; changing the repetition rate from thefirst repetition rate to a second repetition rate in the range of 10kHz-2 MHz without essentially changing an optical setup of the laserengine; and outputting the laser beam of femtosecond pulses with thesecond repetition rate and with a pulse duration less than a 1,000femtoseconds.

In some implementations the amplifying step includes utilizing adispersion compensator in the amplifier to reduce a dispersion of theamplified stretched pulses, caused by an optical component of theamplifier.

In some implementations the reducing the dispersion step includesintroducing a compensating dispersion by at least one chirped mirror inthe amplifier, wherein the compensating dispersion is essentially equalin magnitude and opposite in sign to a dispersion introduced by alloptical elements of the amplifier other than the dispersion compensatorper roundtrip.

In some implementations the changing the repetition rate step includeschanging a number of roundtrips in the amplifier while keeping anoptical setup of the amplifier essentially unchanged.

In some implementations the stretching step and the compressing step areexecuted by the same stretcher-compressor.

In some implementations outputting the laser beam with the secondrepetition rate within one of 1-120 seconds, 10-60 seconds or 20-50seconds after having finished the outputting the laser beam with thefirst repetition rate.

In some implementations changing the repetition rate from the firstrepetition rate to the second repetition rate in a changing time in therange of 1 μs-1 s.

In some implementations a laser engine includes an oscillator thatgenerates a pulsed light beam with a pulse duration of less than 1000femtoseconds; a stretcher-compressor that stretches the duration of thepulses of the light beam; and an amplifier that amplifies an amplitudeof the stretched light pulses to generate amplified stretched pulses,wherein the stretcher-compressor compresses a duration of the amplifiedstretched pulses, and outputs a beam of laser pulses; and the laserengine is operable to output the beam of laser pulses with a firstrepetition rate in the 10 kHz-2 MHz range and subsequently with a secondrepetition rate in the 10 kHz-2 MHz range, utilizing essentially thesame setup of all optical elements of the laser engine, a duration ofthe laser pulses being less than 1000 femtoseconds for the first and thesecond repetition rates.

In some implementations the amplifier includes a dispersion compensatorthat at least partially compensates a dispersion introduced by opticalelements of the amplifier.

In some implementations the amplifier includes a switchable polarizerbetween end-mirrors of the amplifier that switches between a state inwhich the switchable polarizer adjusts a polarization of the amplifiedstretched pulses; and a state in which the switchable polarizer does notadjust the polarization of the amplified stretched pulses with a risetime of less than one of 5 nanoseconds, 4 nanoseconds and 3 nanoseconds.

In some implementations the amplifier includes at least one focusingmirror; and a gain crystal, located near a focal point of the focusingmirror.

In some implementations the laser engine switches between the firstrepetition rate and the second repetition rate in a time less than oneof 60 seconds, 1 second and 10 μs.

In some implementations a laser engine includes an oscillator thatoutputs femtosecond seed pulses; a stretcher that stretches a durationof the seed pulses; an amplifier that amplifies the stretched seedpulses into amplified stretched pulses, and includes a dispersioncompensator to compensate a dispersion of the amplified stretched pulsesinduced by optical elements of the amplifier; and a compressor thatreceives the amplified stretched pulses, compresses a duration of theamplified stretched pulses, and outputs a laser beam of femtosecondpulses.

In some implementations a variable repetition rate laser engine includesa Q-switched cavity dumped regenerative amplifier; the amplifierincluding two end-mirrors, wherein the laser engine outputs femtosecondlaser pulses; and a length of an optical pathway between the end-mirrorsis less than 2 meters.

In some implementations the length of the optical pathway between theend-mirrors is less than 1 meter.

In some implementations the laser engine includes an oscillator thatgenerates seed pulses which are transmitted to the amplifier, wherein alength of a total free-space optical path length from the point wherephotons of the seed pulses are generated in the oscillator to the pointwhere the laser engine outputs the laser pulses is less than one of 500meters, 300 meters, and 150 meters.

In some implementations all edge sizes of a cavity of the amplifier areless than one of 1 meter or 0.5 meter, wherein the cavity of theamplifier houses all optical elements of the amplifier.

In some implementations a footprint of the amplifier is less than one of1 m² or 0.5 m².

In some implementations the laser engine includes a stretcher-compressorthat includes a chirped volume Bragg grating.

In some implementations the amplifier includes a dispersion compensatorthat compensates a dispersion introduced by optical elements of theamplifier.

In some implementations the amplifier includes a laser crystal thatamplifies an amplitude of lasing pulses; and two folding mirrors thatfold a resonant optical pathway inside the amplifier, wherein at leastone of the two end-mirrors and the two folding mirrors is a chirpedmirror.

In some implementations the laser engine is configured to output a laserbeam with a first repetition rate in a repetition rate range; andsubsequently with a second repetition rate in the repetition rate rangewith essentially the same setup of all optical elements of the laserengine.

In some implementations the first and second repetition rates fallwithin a range of one of 10 kHz-2 MHz, 50 kHz-1 MHz or 100 kHz-500 kHz.

In some implementations the laser engine is configured so that the firstrepetition rate is changeable to the second repetition rate in a timeless than one of 60 seconds, 1 second and 1 μs.

In some implementations the amplifier includes a switchable polarizerbetween the end-mirrors that switches in less than one of 5 ns, 4 ns, or3 ns between a state in which the switchable polarizer adjusts apolarization of amplified pulses; and a state in which the switchablepolarizer essentially does not adjust the polarization of the amplifiedpulses.

In some implementations the amplifier includes at least one focusingend-mirror; and a laser crystal, located in close proximity of a focalpoint of the focusing end-mirror.

In some implementations the laser engine includes an oscillator thatgenerates and outputs a beam of femtosecond seed pulses; astretcher-compressor that stretches a duration of the seed pulses; andan amplifier that receives the stretched seed pulses from thestretcher-compressor, amplifies an amplitude of selected stretched seedpulses to create amplified stretched pulses, and outputs a laser beam ofamplified stretched pulses; wherein the stretcher-compressor receivesthe laser beam of amplified stretched pulses, compresses a duration ofthe amplified stretched pulses, and outputs a laser beam of femtosecondpulses with a pulse duration of less than 1,000 femtoseconds; wherein alength of an optical path length from the point where photons of theseed pulses are generated in the oscillator to the point where the laserengine outputs the laser pulses is less than 500 meters.

In some implementations the length of the optical path is less than 300meters.

In some implementations a variable repetition rate laser engine includesan oscillator that generates and outputs a beam of femtosecond seedpulses; a stretcher-compressor that stretches a duration of the seedpulses; and a chirped pulse amplifier that amplifies an amplitude ofselected stretched seed pulses to create amplified stretched pulses;wherein the amplifier includes a switchable polarizer with a switchingtime of less than 5 ns; the stretcher-compressor compresses a durationof the amplified stretched pulses to femtosecond values; and the laserengine occupies an area of less than 1 m².

In some implementations the laser engine is part of a surgical lasersystem, the surgical laser system having the laser engine and an imagingsystem on a top deck of the surgical laser system.

In some implementations a variable repetition rate laser engine includesan oscillator that generates and outputs a beam of femtosecond seedpulses; an integrated stretcher-compressor that stretches a duration ofthe seed pulses; and a Q-switched cavity dumped regenerative amplifierthat amplifies an amplitude of selected stretched seed pulses to createamplified stretched pulses; wherein the stretcher-compressor compressesa duration of the amplified stretched pulses to output femtosecond laserpulses, and a number of optical elements of the laser engine is lessthan 75.

In some implementations the number of optical elements of the laserengine is less than 50.

In some implementations the number of optical elements of the laserengine in portions other than an oscillator is less than 50.

In some implementations the number of optical elements of the laserengine in portions other than the oscillator is less than 35.

In some implementations an optical element is one of: a mirror, a lens,a parallel plate, a polarizer, an isolator, any switchable opticalelement, a refractive element, a transmissive element, or a reflectiveelement.

In some implementations an optical element has the light entering fromair and exiting into air.

In some implementations the integrated stretcher-compressor includes achirped volume Bragg grating.

In some implementations the amplifier includes a dispersion compensatorthat compensates a dispersion introduced by optical elements of theamplifier.

In some implementations the amplifier includes two end-mirrors, defininga resonant cavity; and two folding mirrors that fold a resonant opticalpathway inside the amplifier, wherein at least one of the twoend-mirrors and the two folding mirrors is a chirped mirror.

In some implementations the laser engine is configured to output a laserbeam with a first repetition rate in a repetition rate range; andsubsequently with a second repetition rate in the repetition rate rangewith essentially the same setup of all optical elements of the laserengine, wherein the first and second repetition rates are within a rangeof one of 10 kHz-2 MHz, 50 kHz-1 MHz, or 100 kHz-500 kHz.

In some implementations the laser engine is configured so that the firstrepetition rate is changeable to the second repetition rate in achanging time less than 1 second.

In some implementations the amplifier includes a switchable polarizerbetween the end-mirrors that can switch in less than one of 5 ns, 4 ns,and 3 ns between a state in which the switchable polarizer adjusts apolarization of the amplified stretched pulses; and a state in which theswitchable polarizer essentially does not adjust the polarization of theamplified stretched pulses.

In some implementations the amplifier includes at least one focusingmirror; and a laser crystal, located in close proximity of a focal pointof the focusing mirror.

In some implementations a laser engine includes an oscillator thatgenerates and outputs a beam of femtosecond seed pulses; astretcher-compressor that stretches a duration of the seed pulses; andan amplifier that receives the stretched seed pulses from thestretcher-compressor, amplifies an amplitude of selected stretched seedpulses to create amplified stretched pulses, and outputs the amplifiedstretched pulses; wherein the stretcher-compressor receives theamplified stretched pulses, compresses a duration of the amplifiedstretched pulses, and outputs a laser beam of femtosecond pulses with apulse duration of less than 1,000 femtoseconds; wherein a number ofoptical elements of the laser engine in portions other than theoscillator is less than 50.

In some implementations a number of optical elements of the laser engineis less than 75.

In some implementations a method of scanning with a laser systemincludes the steps of generating laser pulses having a variablerepetition rate with a laser engine; focusing the laser pulses to afocus spot in a target region with a scanning laser delivery system;scanning the focus spot with a scanning speed in the target region withthe scanning laser delivery system; changing the scanning speed; andadjusting the repetition rate according to the changed scanning speedwith a repetition-rate controller.

In some implementations the generating step includes generatingfemtosecond seed pulses by an oscillator; stretching the seed pulses bya stretcher-compressor; amplifying selected stretched seed pulses intoamplified stretched pulses by an amplifier; and compressing theamplified stretched pulses into femtosecond laser pulses by thestretcher-compressor.

In some implementations the method includes adjusting the repetitionrate to approximately maintain a density of laser-generated bubbles inthe target region around a selected value.

In some implementations the density of bubbles is one of a lineardensity, an areal density or a volume density.

In some implementations the adjusting the repetition rate step includesadjusting the repetition rate proportionally to the scanning speed.

In some implementations the adjusting the repetition rate step includesadjusting the repetition rate from a first value to a second value in atime in the range of 1 μsec-1 sec.

In some implementations the scanning the focus spot step includesscanning the focus spot along a minimal acceleration path.

In some implementations the method includes XY scanning the focus spotalong a switchback path; and slowing down the repetition rate whenapproaching the switchback portion of the path.

In some implementations the method includes scanning the laser beamalong a spiral; and slowing down the repetition rate when the scanningapproaches the center of the spiral.

In some implementations the adjusting the repetition rate includesreceiving information by the repetition-rate controller about thechanged scanning speed by one of sensing the changing scanning speed,and getting electronic information about the changing scanning speedfrom a processor or a memory and adjusting the repetition rate accordingto the received information about the changed scanning speed.

In some implementations a variable repetition rate laser scanning systemincludes an oscillator that generates and outputs a beam of femtosecondseed pulses; a stretcher-compressor that stretches a duration of theseed pulses, receives amplified stretched pulses from an amplifier,compresses a duration of the amplified stretched pulses, and outputs alaser beam of femtosecond pulses with a repetition rate; the amplifierthat receives the stretched seed pulses from the stretcher-compressor,amplifies an amplitude of selected stretched seed pulses to createamplified stretched pulses, and outputs the amplified stretched pulsestowards the stretcher-compressor; and a scanning optics that scans afocal spot of the laser beam in a target region with a variable scanningspeed to generate spots of photodisruption; wherein the laser scanningsystem changes the repetition rate to create the spots ofphotodisruption with a predetermined density profile.

In some implementations the amplifier includes a dispersion compensatorthat reduces a dispersion of the amplified stretched pulses.

In some implementations the amplifier includes a switchable polarizerthat rotates a polarization plane of the stretched pulses in theamplifier, wherein a rise time of the switchable polarizer is less thanone of 5 ns, 4 ns, or 3 ns.

In some implementations the laser scanning system includes a controlelectronics that applies control signals to the switchable polarizer tocause the switchable polarizer to switch with a rise time of less thanone of 5 ns, 4 ns, and 3 ns.

In some implementations a method of scanning with a laser engineincludes the steps of generating femtosecond laser pulses with arepetition rate; focusing the laser pulses to a focus spot in a targetregion to generate spots of photodisruption; scanning the focus spot inthe target region with a scanning speed; and adjusting the repetitionrate during the scanning to create spots of photodisruption with adensity profile.

In some implementations the adjusting step includes creating the spotsof photodisruption with one of a linear spot density, an areal spotdensity and a volume spot density being kept essentially even in atarget region.

In some implementations the adjusting step includes adjusting therepetition rate according to a variation of the scanning speed.

In some implementations the adjusting step includes adjusting therepetition rate proportionally to the scanning speed.

In some implementations the adjusting the repetition rate step includesadjusting the repetition rate from a first value to a second value in atime in the range of 1 μsec-1 sec.

In some implementations the generating step includes generatingfemtosecond seed pulses by an oscillator; stretching the seed pulses bya stretcher-compressor; amplifying selected stretched seed pulses intoamplified stretched pulses by an amplifier; and compressing theamplified stretched pulses into femtosecond laser pulses by thestretcher-compressor.

In some implementations the scanning the focus spot step includesscanning the focus spot along a minimal acceleration path.

In some implementations the method includes scanning the focus spotalong a switchback path; and slowing down the repetition rate whenapproaching the switchback portion of the path.

In some implementations the method includes scanning the laser beamalong a spiral; and slowing down the repetition rate according to thescanning approaching the center of the spiral.

In some implementations the method includes scanning the laser beamalong one of an end of a line and a corner of a line; and slowing downthe repetition rate according to the scanning approaching one of the endof the line and corner of the line.

In some implementations the method includes receiving stored or sensedinformation about the scanning speed, and adjusting the repetition rateaccording to the received information regarding the scanning speed.

In some implementations the method includes receiving sensed or imagedinformation about the target region, and adjusting the repetition rateaccording to the received information regarding the target region.

In some implementations, a laser engine can include an oscillator thatoutputs femtosecond seed optical pulses and an amplifier that amplifiesseed optical pulses to produce amplified optical pulses. This amplifierincludes an optical cavity that is coupled to receive and circulate theseed optical pulses, and an optical switch device coupled to the opticalcavity to control coupling of the light of the received seed opticalpulses into the optical cavity and to control coupling of light insidethe optical cavity out as output light of the amplifier. The opticalswitch device is configured to control and adjust a number of roundtripsof the light coupled inside the optical cavity to control and adjust apulse repetition rate of the amplified optical pulses produced by theamplifier. The amplifier also includes an optical gain medium inside theoptical cavity to amplify the seed optical pulses into amplified opticalpulses, and a dispersion compensator inside the optical cavity tocompensate a dispersion of the amplified optical pulses induced by theamplifier. The laser engine includes one or more optical elementsoutside the amplifier to stretch a duration of the seed optical pulsesbefore each seed optical pulse is coupled into the amplifier and tocompress a duration of the amplified optical pulses outputted by theamplifier to produce the amplified optical pulses. The laser engine canbe configured to be free of a dispersion compensation device outside theamplifier that is provided for compensating the dispersion of theamplified optical pulses induced by the amplifier.

In yet other implementations, a method for operating a laser engine toproduce femtosecond optical pulses can include stretching femtosecondseed optical pulses to produce stretched seed optical pulses withreduced optical power in each pulse; and coupling the stretched seedoptical pulses into an optical cavity of an optical amplifier to amplifyoptical power of each stretched seed optical pulse to produce amplifiedstretched optical pulses. Inside the optical amplifier, an opticalcompensator is used to provide dispersion compensation to each opticalpulse, where the optical compensator is structured to introduce adispersion that is opposite in sign and is substantially equal inmagnitude with a dispersion induced by the amplifier within oneroundtrip of light inside the optical cavity of the amplifier, excludingthe dispersion caused by the dispersion compensator. This methodincludes operating an optical switch device coupled to the opticalcavity to control coupling of light of the stretched seed optical pulsesinto the optical cavity and coupling of light of the amplified stretchedoptical pulses out of the optical cavity; compressing a pulse durationof the amplified stretched optical pulses out of the optical cavity toproduce compressed amplified optical pulses as output of the laserengine; and operating the optical switch device to control and adjust anumber of roundtrips of light inside the optical cavity to control andadjust a pulse repetition rate of the compressed amplified opticalpulses, without using a dispersion compensation device, that is locatedoutside the amplifier, to compensate the dispersion induced by theamplifier.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-B illustrate two embodiments of a high power femtosecond laserengine 1.

FIG. 2 illustrates an embodiment of the high power femtosecond laserengine 1 in more detail.

FIG. 3A illustrates the concept of chirping a laser pulse.

FIG. 3B illustrates an example of a stretcher 200′ and a compressor 400.

FIG. 3C illustrates an implementation of an integratedstretcher-compressor 200.

FIG. 4 illustrates an embodiment of an amplifier 300.

FIGS. 5A-B illustrate the pump-gain-dump cycle of a laser cavity.

FIGS. 6A-D illustrate scanning surgical patterns with constant andvariable repetition rates.

FIGS. 7A-B illustrate design challenges relating to thermal lensing attwo different temperatures of the laser crystal 310 in the amplifier300.

FIGS. 7C-D illustrate two implementations of the amplifier 300 withreduced thermal lensing.

FIG. 8 illustrates the dependence of the beam optical power as afunction of the operating temperature.

DETAILED DESCRIPTION

In early femtosecond lasers the extreme shortness of the pulse lengthlead to an extreme high power in these pulses. This high power, however,threatened to damage the gain medium of the lasers. The solution arrivedin the form of chirped pulse amplification (CPA). In this technologyfemtosecond seed pulses are generated, then the length of the seedpulses is stretched by a factor of 10-1000 to the picosecond range, thusdrastically reducing the power within a pulse. These stretched pulsescan be safely amplified with the gain medium without causing damage. Theamplification is followed by a compression, compressing the length ofthe amplified pulses back to femtoseconds. This CPA approach has beenintroduced into numerous applications today.

However, CPA lasers have drawbacks as well. Typically, these lasers havea large number of optical elements and are correspondingly quitecomplex. These factors make the frequency of malfunction quite high andreduce the number of times the lasers can be reliably switched on andoff. Also, the unusually large size of the CPA lasers makes theirintegration into medical devices very challenging, since those aretypically used in the confined spaces of surgical suites or operatingrooms. Moreover, if different procedures call for changing therepetition rate of the pulses, this change requires performingtime-consuming readjustments of the large number of optical elements. Inaddition, thermal lensing impacts the optical performance of most CPAlasers substantially, making them quite sensitive to the operating powerof the laser. This sensitivity is a further obstacle against repetitionrate changes.

Laser designs and techniques for constructing and operating femtosecondpulse lasers described in this document can be implemented to addressvarious technical issues in other femtosecond pulse lasers as well.

FIG. 1A illustrates a chirped pulse amplification (CPA), or cavitydumped regenerative amplifier (CDRA) laser engine 1, which includes anoscillator 100, a stretcher-compressor 200, and an optical amplifier300.

The oscillator 100 can generate and output a beam of femtosecond seedpulses. The stretcher-compressor 200 can stretch a duration of theseseed pulses. The amplifier 300 can receive the stretched seed pulsesfrom the stretcher-compressor 200, amplify an amplitude of the stretchedpulses, and output a laser beam of amplified stretched pulses. Theseamplified stretched pulses can be optically coupled back into thestretcher-compressor 200, which can compress a duration of the amplifiedstretched pulses and output a laser beam of femtosecond pulses.

FIG. 1B illustrates an example of another CPA laser engine 1′ where anoptical amplifier 300′ downstream from an optical oscillator 100′ and anoptical pulse stretcher 200′ can optically couple the amplifiedstretched pulses into a separate compressor 400, which can compress theamplified stretched pulses and output a laser beam of femtosecondpulses.

The description of the laser engines 1 and 1′ contains many controlfunctions and method steps. These functions and steps can be controlledby one or more controllers, processors and other computer-controllers.These controllers, processors and computer-controllers can utilizeadvanced software, interacting with each other. For clarity ofpresentation, these processors, controllers and their correspondingsoftware are suppressed in the figures of this patent document, but aremeant to be part of the description of the laser engines 1 and 1′ insome implementations.

While several of the examples in this application will be described interms of ophthalmic applications, such as cataract surgery, capsulotomyor corneal procedures, implementations of the laser engine 1 can be usedin a remarkably wide range of applications, which include a wide varietyof ophthalmic procedures, such as retinal and corneal surgery, as wellas dermatological and dental applications, different surgicalapplications, and various material machining applications, which shape apiece of material with laser photodisruption or some other laser aidedprocess.

As indicated above, there are various shortcomings of some chirped pulseamplification CPA/CDRA laser engines. Embodiments of the laser engine 1can be configured to offer solutions to these problems by employing someor all of the following design principles as well as other designconsiderations:

(1) Many lasers have a large number of optical elements, such as ahundred or more, making their design complex and pricey. In thiscontext, embodiments of the laser engine 1 can have as few as 50 opticalelements altogether, and no more than 35 optical elements outside theoscillator 100.

(2) Lasers with a large number of optical elements and with thecorresponding complexity can have a high frequency of malfunctions. Insome CPA/CDRA lasers the probability of malfunction can became quitehigh after the laser was “cycled”, i.e. switched on and off 30-40 times.Such systems may require preventive maintenance after 30-40 switchingcycles or more often to preempt an actual malfunction from occurringduring the regular operation of the laser.

In this context, because of the much-reduced number of optical elementsand novel dispersion control solutions, embodiments of the laser engine1 can be cycled 100, 120 or more times with the expectation of regularoperation, thus greatly reducing the frequency of required servicing andincreasing overall reliability.

(3) The large physical extent and the corresponding long duration of theroundtrips of some CPA/CDRA lasers translates to long recharge times asdescribed below, thus limiting their repetition rates, as well as theirutility for being used in space limited surgical devices.

In this context, embodiments of the laser engine 1 can have a compactresonant cavity, which can have an end-mirror-to-end-mirror opticalpathway shorter than one meter in some embodiments and two meters inothers. The compactness is also a factor contributing to the highrepetition rates of the laser engine 1, which can be as high as 300, 500or even 1,000 kHz.

The above compactness can translate to an overall optical pathwaymeasured from the point of generation of the photon to the point of exitand including all the roundtrips in the cavity, to be as low as 150meters in spite of the high repetition rate of these lasers.

(4) Some CPA/CDRA lasers are finely tuned for operating at a specificrepetition rate. This tuning can involve compensating the dispersion ofthe stretcher 200 and the amplifier 300 at the specific repetition rateby the compressor 200/400. However, if an application calls for changingthe repetition rate, the stretcher and the amplifier causes a differentdispersion at this new repetition rate, upsetting the finely tuneddispersion-compensation of the CPA/CDRA laser. To compensate thischanged dispersion, typically the optical elements of the stretcher 200and the compressor 200/400 need to be readjusted in a time-consumingprocedure. This readjustment makes it technically cumbersome to changethe repetition rate of these CPA/CDRA lasers on the time scales of theophthalmic surgical procedures. Therefore, most commercial ophthalmicCPA lasers do not offer the functionality of a variable repetition rate,and none offer a changeable repetition rate during surgical procedures.

In this context, embodiments of the laser engine 1 can include adispersion controller or dispersion compensator that can reduce and evenminimize a dispersion of the laser beam caused by the amplifier 300.This minimization of the dispersion allows the changing of therepetition rate without a time-consuming readjustment of the opticalelements of the laser engine 1. Therefore, the inclusion of thedispersion controller makes it possible to change the repetition rateduring time sensitive surgical procedures. An example is to use a firstrepetition rate for a cataract surgery and a second repetition rate fora capsulotomy or a corneal procedure. As it is well known, in thesesurgeries the time factor is quite crucial.

(5) In some cases, within a surgical procedure cut-patterns may be usedto place the laser spots with an uneven density when the laser beam hasfixed repetition rates. Examples include slowing down a scanning speedaround a turning point of a raster or scanning pattern, or in anarrowing or a broadening spiral.

In this context, embodiments of the laser engine 1 can be configured tohave an essentially continuously adjustable repetition rate and toadjust the repetition rate near-synchronously with the changing scanningspeed to compensate the variations of the scanning speed, allowing theformation of laser spots with a near constant density or with apredetermined density profile.

(6) In addition, thermal lensing negatively impacts the opticalperformance of some CPA/CDRA lasers and makes them undesirably sensitiveto changes in the power and repetition rate of the laser beam. In thiscontext, embodiments of the laser engine 1 can utilize thermal lensingcompensation techniques, making these embodiments quite insensitive tochanges in the power and repetition rate of the applied laser beam.

FIG. 2 illustrates a specific implementation of the laser engine 1 indetail. The oscillator 100 can be a wide variety of light sources whichcan generate and output seed pulses for the laser engine 1. Examplesinclude diode pumped fiber oscillators. The oscillator may include asingle diode, e.g. a GaAs diode operating at an 808 nm wavelength, or alarge variety of other diodes.

Fiber oscillators are much smaller than oscillators based on free spacebeam propagation. In surgical applications, where the crowdedness of theoperating theatre is a pressing constraint, reducing the spatial extentof the laser engine is a highly prized design feature.

In some examples, the oscillator outputs high quality seed pulses.Several factors can contribute to the high pulse quality as detailednext.

(i) In some embodiments the diode can include a frequency stabilizingbar, such as a volume Bragg grating inside the diode. Such gratings canprovide pulses with low noise and high pulse-to-pulse stability. Thefiber may be formed of glass doped by Nd or Yb.

(ii) The oscillator 100 can include a semiconductor saturable absorbermirror, or SESAM. Utilizing one or more SESAMs improves the coherence ofthe modes within the generated pulses, resulting in an essentiallymode-locked operation.

Oscillators with the above design principles can output essentiallytransform-limited seed pulses, e.g. with a Gaussian shape. In someexamples, flat-top pulses may be also generated. The pulse-duration canbe less than 1,000 femtoseconds (fs). In some implementations, the pulseduration can be in the 50-1,000 femtoseconds range, in some otherembodiments in the 100-500 femtoseconds range. The seed pulse frequency,or repetition rate can be in the range of 10-100 MHz, in otherembodiments in the range of 20-50 MHz. Decreasing the seed pulsefrequency below 10 or 20 MHz raises a series of design challengesthough. For this reason, most oscillators operate at frequencies above20 MHz.

The power of the beam of seed pulses can be in the range of 10-1000 mW,in other embodiments in the range of 100-200 mW.

For many timing considerations, the oscillator 100 can be used as amaster clock.

The stretcher-compressor 200 can stretch the seed pulses by introducingdifferent delay times for the different frequency-components of thepulse. In short, the stretcher-compressor can introduce a dispersion orchirp.

FIG. 3A illustrates this chirp in detail. The stretcher-compressor 200may receive a short pulse, whose frequency content, or spectrum, isapproximately uniform, or “white”, across most of the duration of thepulse. In other words, the amplitude of the different frequencycomponents at the beginning of the pulse is approximately even andremains so during the duration of the pulse. The stretcher-compressor200 can stretch the pulse length by introducing different delay timesfor the red, green and blue components of such “white” pulses.Therefore, the frequency content, or spectrum, of the pulse outputted bythe stretcher-compressor 200 can become time dependent. According to atypical convention, pulses where the leading part is dominated by thered frequencies while the trailing portion is dominated by bluefrequencies are referred to as having a positive dispersion or chirp.

The present description refers to chirp in the time domain, i.e. to therelative delay of the high and low frequency components. Spatial chirp,i.e. the separation of high and low frequency components spatiallywithin the beam raises a variety of additional design challenges and isnot among the desired functionalities of the stretcher 200′ orstretcher-compressor 200.

The stretcher-compressor 200 or the stretcher 200′ can introduce apositive chirp into initially white seed pulses by enhancing the redcontent in the leading portion of the pulse and enhancing the bluecontent in the trailing portion of the pulse. Analogously, non-whitepulses can also be chirped by the stretcher-compressor 200 or thestretcher 200′.

The stretcher-compressor 200 may stretch a duration of the femtosecondseed pulses from a range of 50-1,000 femtoseconds to a stretchedduration of 1,000-200,000 femtoseconds, or 1-200 picoseconds or even upto 500 ps. The stretcher-compressor 200 can stretch a duration of thefemtosecond seed pulses by a factor greater than 10. In some cases, thestretching factor can be greater than 10², 10³, 10⁴, or 10⁵. Each ofthese stretching factors introduces different design criteria for theamplifier 300.

FIG. 3B illustrates that the laser engines 1′ of the type shown in FIG.1B can utilize a stretcher 200′ and a separate compressor 400. Thestretcher 200′ can include a first grating 201, a lens 202, a secondgrating 203, and a mirror 204. When a short pulse 211 enters thestretcher 200′, the first grating 201 can refract the differentfrequency components into different directions. Upon exiting the firstgrating 201, the diverging rays may propagate to the lens 202 and getredirected to the second grating 203. Some embodiments may use twolenses in place of the lens 202. Since the second grating 203 makes anangle with the first grating 201 and the different frequency rayspropagate in diverging directions, the different frequency componentstravel different distances, needing different times to do so.

For example, in the stretcher 200′ of FIG. 3B the components withfrequencies in the blue region of the spectrum travel a longer distancethan the components in the red region, acquiring a delay relative to thered component of the incident short pulse. Here and throughout, theterms “blue” and “red” are used in an illustrative and relative manner.They refer to the components of the pulse spectrum with shorter andlonger wavelengths, respectively. In particular implementations, thelaser mean wavelength can be in the 1000-1100 nm and the bandwidth ofthe pulse can be in the range of 2-50 nm, in some cases in the range of5-20 nm. In this example the entire spectrum of the pulse is in theinfrared region. In this example, the terms “blue” and “red” refer tothe portions of the infrared spectrum which have shorter and longerwavelengths within the bandwidth of the pulse, respectively.

The functions of the second grating 203 include the partial control ofthe chirp, i.e. the delay of the blue component relative to the redcomponent as well as the restoration of the beam to an essentiallyparallel beam to make it suitable for reflection by the mirror 204. Themirror 204 reflects the frequency-separated parallel rays, which thenretrace their optical path through the second grating 203, the lens 202and the first grating 201. By the time the pulse exits the first grating201, the blue component of the pulse travels considerably longerdistance and thus lags behind the red component.

This delay has at least three effects on the outputted pulse: (i) thepulse length gets considerably longer, (ii) the amplitudes of thedifferent frequency-components are shifted relative to one another intime, shifting the red components to the leading edge of the pulse andthe blue components to the trailing edge, or vice versa, (iii) the totalenergy of the pulse is distributed over a longer pulse length, reducingthe optical power of the outputted pulse. In some cases, the pulseduration can be stretched by a factor of 100, 1000 or more, the powercorrespondingly can be reduced by a factor of a 100, 1000, or more. Insum, the stretcher-compressor 200 or the stretcher 200′ can stretch thepulse, introduce a positive chirp and thereby substantially reduce thepower of the pulse.

As described earlier, reducing the peak power of the pulse is abeneficial aspect of the CPA/CDRA lasers as the cavity optics of thesubsequent amplifier 300 are not exposed to pulses of excessively highpower and thus avoid getting damaged by the beam.

FIG. 3B also illustrates an example of a compressor 400, which caninclude a third grating 205, a fourth grating 207 and a mirror 208. Someexamples have no lens between these gratings, while others may have oneor two lenses. The third grating 205 again directs different componentsof the pulse spectrum in different directions in analogy with the firstgrating 201 of the stretcher 200′. The fourth grating 207 againpartially controls the relative delays of the blue and red componentsthrough its orientation, in analogy with the second grating 203.However, since the fourth grating 207 is now oriented opposite to thesecond grating 203, the optical pathway of the blue components is nowshorter, causing a negative chirp. This negative dispersion allows theblue components of the stretched pulse to catch up with the redcomponents, shortening the overall duration of the amplified stretchedpulses from hundreds of picoseconds to hundreds of femtoseconds. Designswith the separate stretchers 200′ and compressor 400 are embodiments ofthe laser engine 1′ of FIG. 1B.

FIG. 3B also illustrates two sensitive aspects of the designs of FIG.1B, having a separate stretcher 200′ and compressor 400.

(i) First, the stretcher 200′, the amplifier 300 and the compressor 400need to be fine tuned with each other, so that the compressor 400 canundo the stretching caused by the stretcher 200′ and the subsequentdispersion caused by the amplifier 300 with high precision. Therefore,setting the location of the lens 202 and the orientation of the first tofourth gratings 201-207 may require especially high precision tocompensate the dispersion of the amplified stretched pulses and tocompress them back to femtosecond pulses. And, of course, high precisionadjustments are quite sensitive to perturbations: small changes intemperature, number of roundtrips, and mechanical stress can underminethe precision adjustment, requiring maintenance and re-calibration ofthe laser engine 1′ with the architecture of FIG. 1B.

(ii) In some complex or multi-step procedures, the change of therepetition rate may be desirable. However, such a change of therepetition rate is typically accompanied by a change of the number ofroundtrips to optimize the outputted pulses. In turn, the change of thenumber of roundtrips often causes a change in the thermal lensing aswell as the compounded dispersion caused by the amplifier 300.Therefore, the change of the repetition rate and the number ofroundtrips can upset the carefully calibrated balance of the stretching,dispersion and compression.

To counteract these changes, as shown by the arrows of FIG. 3B, someimplementations of the laser engine 1′ might be recalibrated by changingthe location of the lens 202, the position or the orientation of some ofthe gratings 201, 203, 205 and 207, the location of the mirrors 204 and208, or the location where the beam hits the lens 202 by moving one ormore mirrors. Needless to say, these changes typically require cautiousand often iterative mechanical adjustments and precision calibration,all of which are time consuming interventions.

The slowness of the recalibration can pose a problem in applicationswhere a timely change of the pulse-repetition rate is desired. This canbe especially prohibitive in applications where time is a criticalfactor, e.g. during ophthalmic surgical applications, where thepatient's ability to control eye movements may be as low as 90 seconds.For all of these reasons, most laser engines do not offer thefunctionality of a changeable repetition rate.

In addition, since in the laser engine 1′ the stretcher 200′ is separatefrom the compressor 400 and both of them include multiple gratings andlenses, the spatial extent of the stretcher and compressor of the laserengine 1′ of the type in FIG. 1B is typically spatially quite extensive.

To reduce the spatial footprint of the stretcher 200′ and the compressor400, as well as to reduce calibration times, in some implementations ofthe laser engine 1′, the stretcher 200′ and the compressor 400 can shareone or more optical elements. In some cases, they can share a grating,such as the first grating 201 and the third grating 205 can be the same.

In some multiply folded examples the two gratings of the stretcher 200′can be the same physical grating, the lenses and mirrors directing thebeam on the same grating from different directions during differentpasses. In some multiply folded examples, all functions of the twogratings of the stretcher 200 and the two gratings of the compressor 400can be performed by a single shared grating.

FIG. 3C illustrates an example of the stretcher-compressor 200 of theembodiment of FIG. 1A, which offers a robust solution to thesechallenges. The stretcher-compressor 200 of FIG. 3C integrates thestretching and the compressing functionalities, and thus it can beemployed in an embodiment of the laser engine 1 of FIG. 1A. Thisstretcher-compressor 200 as implemented in the example in FIG. 3C is achirped volume Bragg grating (CVBG). This CVBG can be a stack of layers,e.g., in a photothermal refractive (PTR) glass, the layers havingsuitable indices of refraction and a grating period that varies with theposition of the layers. In such a design the Bragg resonance conditionoccurs at different positions for different spectral components of apulse. Thus, different spectral components are reflected at differentlocations, acquiring different time delays within the pulse.

As shown in the example in FIG. 3C, when a short “white” pulse 211enters the stretcher-compressor 200, the red frequency components getrefracted from the near regions with wider layer spacings or gratingperiods, since their wavelength is longer and satisfies the Braggreflection conditions in these near regions. In contrast, the bluefrequency components, having shorter wavelengths, are returned from thefarther regions of the grating. Since the blue components traverse alonger optical path, they acquire a delay relative to the redcomponents. Thus, the inputted short white pulse 211 is stretched bythis CVBG stretcher-compressor 200 into a longer stretched pulse 212. Inthe specific example, the stretched pulse 212 develops a positive chirpbecause the blue components are delayed relative to the red components.Other implementations can have a CVBG producing a negative chirp,delaying the red spectral components relative to the blue ones.

This CVBG stretcher-compressor 200 can also compress the amplifiedstretched pulses 213 with high precision without any cumbersome finetuning, since the stretched pulses, after amplification by the amplifier300, are injected into the same CVBG stretcher-compressor 200 from theopposite end, or compressor port. When a stretched pulse enters the CVBGstretcher-compressor 200 from the opposite end, its red components aredelayed to the same degree as its blue components were delayed duringthe stretching step, restoring the original short length of the pulse.Therefore, this stretcher-compressor 200 can compensate the dispersionintroduced during the stretching very efficiently and output a properlycompressed amplified pulse 214.

In comparison to the particular aspects of laser engines 1′ withseparated stretcher 200′ and compressor 400, (i) the laser engine 1 isnot highly sensitive to the precise alignment of moving optical elementssince it has none, and thus shows a remarkable robustness againstmechanical perturbations or changes of the operating temperature, and(ii) since the novel design of the amplifier 300 does not induceadditional dispersion in relation to the number of roundtrips asexplained further in relation to Eqs. (1)-(2) and FIGS. 5A-B, the laserengine 1 does not require sensitive recalibration and re-alignments ofits optical elements and setup when the repetition rate is changed.These attributes enable the use of the laser engine 1 in applicationswhere a fast or timely change of the repetition rate is important.

In other designs different from what is described above, the amplifier300 can introduce additional dispersion. In these designs the integratedarchitecture of the stretcher-compressor 200 can be supplemented with are-adjusting functionality as the compressor has to compress not onlythe dispersion of the stretcher, but the additional dispersion of theamplifier 300. This added task might require implementing a tunableblock in relation to the compressor functionality.

Returning to FIG. 2, the laser engine 1 can further include an effectivepolarizing beam splitter 150. Beam splitter 150 can include a polarizerand a λ/4 plate between the oscillator 100 and the stretcher-compressor200. In other embodiments, the beam splitter 150 can be a thin filmpolarizer. This combination 150 can let through the seed pulses from theoscillator 100 to the stretcher-compressor 200, but redirect thestretched pulses coming back from the stretcher-compressor 200 towardthe amplifier 300, because the λ/4 plate rotates the polarization planeof the beam of pulses by 90 degrees upon double passing. The polarizer,while transmissive for the polarization direction of the seed pulses, isreflective for the 90 degree rotated polarization plane of the stretchedpulses, after they cross the lambda/4 plate the second time.

In some embodiments, the laser engine 1 can include a Faraday isolator500 in the optical pathway between the beam splitter 150 and theamplifier 300. The functions of the Faraday isolator 500 can include theisolation of the oscillator 100 from the amplified beam in order toprevent damage by the high power of the laser beam to the oscillator100. Such a Faraday isolator 500 can receive the stretched seed pulsesfrom the beam splitter 150, transmit the stretched seed pulses towardthe amplifier 300, receive the laser beam of amplified stretched pulsesfrom the amplifier 300, and output the laser beam of amplified stretchedpulses towards the stretcher-compressor 200 through polarizers 550 and560.

Faraday isolators 500 can be useful in embodiments where the amplifier300 outputs the amplified pulses through the same optical path itreceived them, because simple redirecting optics maybe quite inadequatefor the isolating function as the amplified pulses often have a power orintensity which is hundreds or even thousands of times greater than thatof the seed pulses. Even if the simple redirecting optics lets throughonly a fraction of these amplified pulses, the transmitted pulses canstill be intense enough to damage the oscillator 100.

In some embodiments, the Faraday isolator 500 can be configured to letless than a 1/10,000 portion of the laser beam from the amplifier 300through towards the oscillator 100. The same isolating function can becaptured in terms of attenuation: the Faraday isolator may attenuate theamplified laser beam by e.g. 40 dB or in some implementations by 50 dB.

The Faraday isolator, or polarization dependent isolator, may includethree parts: an input polarizer, polarized vertically, a Faradayrotator, and an output polarizer or analyzer, polarized at 45 degrees.

Light travelling in the forward direction becomes polarized e.g.vertically by the input polarizer, if it wasn't already polarized inthat direction. (Here, the polarization plane refers to the plane inwhich the electrical field vectors lie. Further, “vertical” onlyestablishes a convention or a reference plane. In various embodimentsthe actual polarization plane can be oriented into other specificdirections.) The Faraday rotator rotates the polarization plane of thebeam by about 45 degrees, aligning it with the polarization plane of theanalyzer, which then transmits the light without additional rotation ofthe polarization plane.

Light travelling in the backward direction, such as the amplified pulsesreturning from the amplifier 300, becomes polarized at 45 degreesrelative to the reference vertical plane after exiting the analyzer. TheFaraday rotator again rotates the polarization by about 45 degrees.Therefore, the light outputted by the Faraday rotator towards the inputpolarizer is polarized horizontally. Since the input polarizer isvertically polarized, the horizontally polarized light will be reflectedby the input polarizer with near perfection instead of transmitting itto the oscillator 100. Thus, the Faraday isolator 500 can protect theoscillator 100 from the high energy amplified laser pulses with a highefficiency.

The Faraday rotator typically achieves its function by generating amagnetic field pointing in the direction of the optical axis. SomeFaraday rotators include permanent magnets to achieve thisfunctionality.

The optical materials used in Faraday rotators typically have a highVerdet constant, a low absorption coefficient, low non-linear refractiveindex and high damage threshold. Also, to prevent self-focusing andother heating-related effects, the optical pathway is typically short.The two most commonly used materials for the 700-1100 nanometer rangeare terbium doped borosilicate glass and terbium gallium garnet crystal(TGG).

Embodiments of the laser engine 1 or 1′ where the amplifier 300 does notoutput the amplified pulses via the same optical pathway as they enteredmay not need to employ the Faraday isolator 500.

FIGS. 2 and 4 illustrate that the light transmitted from the Faradayisolator 500 can enter the amplifier 300. The amplifier 300 can includea laser crystal, or gain medium 310 to amplify the stretched seed pulseswhich make roundtrips between end-mirrors 321 and 322. Some amplifiers300 can include a folded optical pathway (or “z-cavity”), redirectingthe laser beam with folding mirrors to reduce the spatial extent of theresonant cavity. The amplifier 300 in FIG. 4 has four mirrors: the twoend-mirrors 321 and 322, which define the resonant cavity, and twofolding mirrors 323 and 324. In some examples, the optical pathway caneven fold over itself, appearing as a crossing pattern. While utilizingmore folding mirrors can reduce the size of the amplifier 300 evenfurther by folding the optical pathway into a more compact space, theadditional mirrors increase the potential for misalignment and theprice.

In addition to the laser crystal 310 and mirrors 321-324, the amplifier300 can include a switchable polarizer 330, which controls the qualityfactor Q and thus the amplifying function of the amplifier 300, as wellas a thin film polarizer 340, which serves as an input/output port forthe pulses in the cavity. The thin film polarizer 340 is a specificexample of a polarization-selective device which reflects light with afirst predetermined polarization, while transmitting light with a secondpolarization that is orthogonal to the first predetermined polarization.The switchable polarizer 330 can be a polarization device that switchesbetween a first operating state when it does not rotate the polarizationof the light passing through it and a second operating state when itrotates the polarization of the light in response to a control signalapplied thereto. The combination of the thin film polarizer 340 and theswitchable polarizer 330 can be used to control when the pulses comingfrom the Faraday rotator 500 are coupled into the amplifier 300, andwhen the pulses amplified inside the amplifier 300 are coupled out fromthe amplifier, as explained below.

This combination of the thin film polarizer 340 and the switchablepolarizer 330 in FIG. 4 is an example of an optical switch for theresonant cavity of the amplifier 300. Other designs can be also used forthis optical switch.

The operation and the structure of the amplifier 300 are described infurther detail below. In particular, it will be shown that changing therepetition rate is often accompanied by changing the number ofroundtrips an amplified pulse makes between the end-mirrors 321 and 322.A function of the just-described optical switch is to control the numberof these roundtrips by controlling when pulses are coupled into or outof the resonant cavity.

The optical elements in the amplifier 300 can introduce a certain amountof dispersion during each of these roundtrips. Thus, changing the numberof roundtrips in the amplifier 300 in relation to changing therepetition rate changes the cumulative dispersion of the amplifiedpulses outputted by the amplifier 300. Even if the compressor 400 isadjusted to compensate the dispersion for a particular number ofroundtrips, the change of the dispersion from the change of the numberof roundtrips upsets the sensitive balance of stretching, dispersiveamplification and compression of the stretcher 200′, the amplifier 300and the compressor 400 of the laser engine 1′ of FIG. 1B, requiringlengthy recalibration. Even the more inventive architecture of the laserengine 1 with the integrated stretcher-compressor 200 in FIG. 1A mayrequire the use of a compensating element to be adjusted when the numberof roundtrips is changed. This aspect limits the utility of these laserengines.

To broaden their utility, some laser engines can include a dispersioncontroller or compensator as part of the amplifier 300. A function ofthe dispersion controller is to introduce dispersion opposite andessentially equal to the dispersion introduced by the optical elementsof the amplifier 300 during a roundtrip. As a result of this dispersioncompensation or control, the pulses acquire little or no dispersionduring the roundtrips in the resonant cavity of the amplifier 300. Thus,changing the number of roundtrips changes the dispersion of theamplified pulses only to a miniscule degree or not at all.

Therefore, the repetition rate of the laser pulses can be varied withessentially no adjustment, re-alignment or calibration of the opticalsetup of the compressor 400 or stretcher-compressor 200 as no dispersionaccumulates during the roundtrips to compensate. Accordingly, thedispersion-controlled amplifier 300 can be implemented in the laserengine 1′ of FIG. 1B to relieve the compressor 400 from the task oftime-consuming realignments upon the change of repetition rates.Moreover, this dispersion-controlled amplifier 300 enables the use ofthe integrated stretcher-compressor 200 in the laser engine 1 of FIG. 1Awithout adjustable compensating functionalities.

For example, if the laser crystal 310 introduces a positive dispersionduring a roundtrip of a lasing pulse inside the resonant cavity, thedispersion controller can introduce a negative dispersion of the samemagnitude to the amplified stretched pulses to suppress, minimize oreliminate the dispersion of the lasing pulse.

A useful measure to quantify the dispersion is the “group delaydispersion”, or GDD, often defined as:

$\begin{matrix}{{G\; D\; D} = {\frac{\lambda^{3}}{c^{2}}\frac{\mathbb{d}^{2}{n(\lambda)}}{\mathbb{d}\lambda^{2}}L}} & (1)\end{matrix}$where λ is the wavelength of the light, c is the speed of light, n(λ) isthe wavelength dependent index of refraction and L is the length of theoptical pathway in the cavity. The GDD of the optical elements 310, 330and 340, the mirrors 321-324, and any other optical element which may bepresent in the amplifier 300 can be determined e.g. by measurement orinferred from the design. Armed with the knowledge of the GDD, adispersion controller can be implemented in the cavity with a GDD ofapproximately equal and opposite value to the determined GDD of theoptical elements of the amplifier 300. The so-designed cavity produceslittle or no dispersion during the roundtrips of the pulses, eliminatingthe described problems and broadening the utility of the laser engines 1or 1′.

In an illustrative example, in a typical CPA laser engine 1′ a 500femtoseconds seed pulse can get stretched by 200 picoseconds to astretched pulse length 200.5 ps by the stretcher 200′. The correspondingcompressor 400 may be adjusted and calibrated to compress the stretchedpulse back by 200 ps, resulting in a compressed pulse length of ideallyabout 500 fs. Accounting for imperfections, in realistic cases thecompressed pulse length may fall in the range of 500-800 fs.

However, during the roundtrips of the stretched pulses in the resonantcavity of the amplifier 300, the length of the stretched pulses may getenhanced by the dispersion of the various optical elements of theamplifier 300, represented by the GDD of the cavity. Typical values ofthe GDD can vary from hundreds of fs² to hundreds of thousand fs². Insome cases the GDD can be within the range of 5,000 fs²-20,000 fs².Since typically the stretcher 200 and the compensator 400 cancel eachother's effect on the pulse length, the length of the pulse Δt(out),outputted by the laser engine 1, is related to the length of the seedpulse Δt(seed), generated by the oscillator 100, and the GDD via thefollowing relation:

$\begin{matrix}{{\Delta\;{t({out})}} = {\frac{\sqrt{{\Delta\;{t({seed})}^{4}} + \left( {4\ln\; 2N \times G\; D\; D} \right)^{2}}}{\Delta\;{t({seed})}} = {\Delta\;{t({seed})}\sqrt{1 + {7.69N^{2} \times \left( \frac{G\; D\; D}{\Delta\;{t({seed})}^{2}} \right)^{2}}}}}} & (2)\end{matrix}$where N is the number of roundtrips in the cavity.

Thus, for example, the length of a Δt(seed)=200 fs seed pulse can beincreased by as little as 22 fs to Δt(out)=222 fs during a singleroundtrip by the optical elements of the amplifier with a GDD of 7,000fs². However, this seemingly small dispersion gets compounded during therepeated roundtrips. After N=10 roundtrips the length of the outputtedpulse increases by about 790 fs to Δt(out)=990 fs, after N=30 roundtripsby about 2,700 fs=2.7 ps to Δt(out)=2,920 fs=2.9 ps, and after N=100roundtrips by about 9.5 ps to Δt(out)=9.7 ps. Visibly, without adispersion controlled amplifier 300 this substantial deterioration ofthe pulse length by a factor of up to about 50 transforms the laser froma femtosecond laser to a picosecond laser.

Further, even if the compressor 200 or 400 is calibrated to compensatethe additional dispersion caused by a specific number of roundtrips,e.g., the 9.5 ps dispersion corresponding to the N=100 roundtrips, whenan application calls for changing the number of roundtrips from N=100to, e.g., N=110, another 1 ps dispersion is induced by the amplifier300, again resulting in a compressed pulse length of picoseconds insteadof femtoseconds.

In contrast, embodiments of the laser engine 1 or 1′ can have adispersion controller inside the amplifier 300 to compensate the GDDcaused by the optical elements of the resonant cavity. This dispersioncontroller can compensate the few fs per roundtrip dispersion induced bythe optical elements in the amplifier. Thus, the amplifier 300 canreceive stretched pulses with a 200 ps pulse-length and emit amplifiedpulses with essentially the same 200 ps pulse-length, approximatelyindependently from the number of roundtrips the amplifier is operatedat, let that number be 50, 100, 200 or 500. Therefore, thestretcher-compressor 200 of the laser engine 1, or the compressor 400 ofthe laser engine 1′, can compress the pulse-length back to thefemtosecond range for a wide range of the number of roundtrips N andhence for a wide range of repetition rates without necessitating thetime-consuming re-adjustment and calibration of other laser systems thatlack the present dispersion control or compensation inside the amplifier300. The dispersion controller inside the amplifier 300 is in theinternal optical path of the amplifier 300 and thus automaticallycompensates the GDD/dispersion without requiring a re-adjustment of theoptical elements outside the optical amplifier 300. With the properdesign of the dispersion controller inside the amplifier 300, the needfor having adjustable dispersion elements outside the optical amplifier,such as the dispersion compensation gratings in FIG. 3B, to bere-adjusted for changing the pulse repetition rate, can be eliminated.

Enabled by the above design considerations, the laser engines 1 or 1′can produce a laser beam with a pulse duration less than 1000femtoseconds with repetition rates in the 10 kHz-2 MHz range withessentially the same setup of all of the optical elements of the laserengine other than those of the oscillator 100. Other embodiments canoperate with a repetition rate in the range of 50 kHz-1 MHz, yet othersin the range of 100 kHz-500 kHz.

Therefore, in these laser engines, the repetition rate can be variedfrom a first value to a second value without changing the setup of theoptical elements of the laser engine other than the oscillator 100.

There can be laser engines where the change of the repetition rates fromits first value to the second value is accompanied by a change of thesetup of the optical elements. However, some of these laser engines maybe modifiable based on dispersion compensation or control inside theiramplifier so that the modified laser engines can be operated to outputthe laser beam with the second repetition rate with an unmodified setupas well.

In various implementations of the laser engine 1 the repetition rate canbe varied from a first value to a second value, where the secondrepetition rate is different from the first repetition rate by at least10%, 50%, 100% or 200%.

In some designs, where the resonant cavity employs optical fibers, anadjustment of the repetition rate may also be possible without retuningand adjusting a subsequent compressor 400. However, these fiber lasers(i) have serious limitations on the energy of the pulses, and (ii) oftendo not have dispersion controllers. They typically produce pulses onlywith energy below 10 micro Joule (μJ) per pulse to avoid the danger ofdamaging the fiber cavity. As described below, for many ophthalmic andsurgical applications this energy per pulse may be insufficient, asthose applications may require 20 or more μJ/pulse on target,translating to 30 or more μJ/pulse outputted by the laser accounting forthe various losses.

Another point of difference is that in fiber lasers the divergence ofthe beam unavoidably changes when the repetition rate of the laserchanges because of the variation of the thermal load.

In contrast, the amplifier 300 typically contains a dispersioncontroller or compensator and the light propagates in free space so thatsome examples of the laser engine 1 or 1′ can be operated to output alaser beam with an energy in the range of 1-100 μJ/pulse, others with anenergy in the range of 10-50 μJ/pulse, yet others with an energy in therange of 20-30 μJ/pulse.

Some laser engines 1 or 1′ may be configured so that the changing of therepetition rate is accompanied with an adjustment of an optical elementof the laser engine 1. However, because of the presence of thedispersion controller, even in these embodiments the laser engine 1 or1′ may be modifiable to utilize essentially the same setup of theoptical elements when the repetition rate is changed.

The above described examples can be implemented in many different ways.In some embodiments the dispersion controller or compensator inside theoptical amplifier can include one or more chirped mirrors, chirpedfibers, various chirped gratings, chirped transmissive optical elements,prisms, and other optical elements, capable of changing the dispersionof the incident light.

In general, chirped optical elements can have a number of layers withmodulated optical properties. In examples, the thickness of the layersand the variation of their refractive index can be designed to controllight with different wavelength differently. An example, the chirpedvolume Bragg grating (CVBG) has been already described in relation tothe stretcher-compressor 200. Other examples, such as chirped mirrorscan include layers of dielectric materials, where each single dielectriclayer, or short stack of layers, can reflect a narrow vicinity of aspecific wavelength. The chirped mirror can be constructed by forming afirst stack of 5-10 dielectric layers with thickness suitable to reflectlight with a wavelength in a vicinity of a first wavelength. Then asecond stack of 5-10 dielectric layers can be formed on top of the firststack with a different thickness and/or index of refraction to reflectlight with a wavelength in the vicinity of a second wavelength and soon. When formed with a sufficient number of layers in a suitable numberof stacks, the chirped mirror can reflect light with wavelengthcomponents in a selected band of wavelengths, while transmitting lightwith other wavelengths.

The dispersion controlling function in the amplifier can be performed bymaking one or more of the mirrors 321-324 chirped. In FIG. 4 all fourmirrors are chirped. Other designs may have only one or two of themirrors chirped. Yet others may employ one or more chirped opticalelement. As possible realizations of the dispersion controller, theseone or more chirped mirrors can control, compensate, minimize, or eveneliminate the dispersion, induced by the optical elements 310, 330 and340 and the mirrors 321-324 during a roundtrip of an amplified stretchedlaser pulses inside the resonant cavity of the amplifier 300.

The laser crystal 310 can be Nd or Yb based. Examples include Nd:YAG andYb:YAG crystals. Other implementations may use Nd or Yb doped glass. Yetothers Yb:tungstates of the form Yb:X(WO₄)₂ or Yb:sesquioxides of theform Yb:X₂O₃. In these cases, X can be Y, Lu, Gd or other suitableelements. The Nd or Yb doping level can be in the range of 0.1-100%.

The spatial doping profile of the laser crystal may be chosen to ensurethe emission of high quality single mode laser pulses. Some dopingprofiles may be compatible with a pumping light source with limitedfocusability as expressed by a larger than usual M² factor of thepumping light. The pumping source can be in a side-pumping or in anend-pumping arrangement. The pumping light source may contain multiplefiber-coupled diodes, such as 2-10 diodes, each emitting with 1-10 W ofpower. The pumping diodes can operate in an essentially continuous wave(CW) operation mode, or in an analogous high frequency pulse mode. Theycan be arranged in different spatial arrays, bars or other forms. Thelight from the diodes can be guided through a shared grating, which mayreturn a very small percent of the light to the diodes, thus phaselocking their light.

FIGS. 5A-B, in combination with FIG. 4, illustrate the operation of thecavity dumped regenerative amplifier CDRA 300. The principle of theoperation is often referred to as “Q-switching”, referring to theswitching of the quality factor Q of the resonant cavity.

In a “recharge”, or “pump” phase, the thin film polarizer 340 reflectsthe incoming light through the switchable polarizer 330. The switchablepolarizer 330 can be a shutter, a chopper wheel, a spinning prism ormirror, an acusto-optic device, an electro-optic device, such as aPockels cell or Kerr cell, or a switchable λ/4 wave-plate. In anunbiased or low voltage state the switchable polarizer 330 can rotatethe polarization plane by 90 degrees as the pulses pass through twice,to and from the end-mirror 322.

During the recharge or pump period the Faraday isolator 500 transmitspulses onto the thin film polarizer 340 which redirects them through theswitchable polarizer 330. Returning from the end-mirror 322 the pulsescross the switchable polarizer 330 for the second time. Then theyperform one roundtrip in the cavity, passing through the switchablepolarizer 330 two more times on their way to and from the end-mirror322. After one roundtrip these four passes through the switchablepolarizer 330 rotate the polarization plane of the pulses by 180degrees. Thus, they get reflected out of the cavity by the thin filmpolarizer 340 essentially without amplification.

In this same recharge or pump period the amplifier 300 suppresses thelasing action of the light generated by the pumping diodes inside thecavity as well, as the 90 degree double pass rotation of thepolarization plane by the switchable polarizer 330 makes the qualityfactor Q of the resonant cavity low, making the cavity unsuitable forlasing action.

FIG. 5A illustrates that in this recharge/pump phase the laser crystal310 absorbs the light from the above described pump diodes, or pumplaser diodes, in a side- or end-pumping arrangement. The pumpingincreases the population of an excited energy level of the lasing atomsor complexes to create population-inversion, in essence absorbing andstoring the pumping energy or “gain”.

FIG. 5B illustrates that in this recharge/pump phase no amplified laserpulses are generated in and emitted by the amplifier 300. The rejectedunamplified pulses, of course, are emitted by the amplifier 300.

FIGS. 5A-B illustrate that the pump/recharge phase can end eitheraccording to a predetermined timing operation or prompted by a sensingelectronics which tracks the energy storage in the laser crystal 310. Ineither case, after a time t(recharge) a control and driver electronicsmay apply a high voltage to the switchable polarizer 330 to stoprotating the polarization plane by 90 degrees. Other types of theswitchable polarizer 330 may be switched by different means. This changeswitches the quality factor Q of the cavity to a sufficiently high valueto make the cavity suitable for lasing action.

Single pulse embodiments of the amplifier 300 can switch the switchablepolarizer 300 while a single pulse is performing its roundtrip insidethe cavity. When at the end of its roundtrip the single pulse returns tothe switchable polarizer 300 after that has switched, the polarizationplane of the pulse is not rotated anymore and therefore the pulse is notreflected out from the cavity by the thin film polarizer 340. Instead ofgetting rejected as during the pump phase, the pulse can be trapped inthe cavity for several more roundtrips for a gain period of lengtht(gain). In FIG. 5B the time scale of t(gain) has been magnified forclarity.

FIGS. 5A-B illustrate that in the gain period the energy (or gain)pumped and stored in the cavity gets transferred from the laser crystal310 to the pulse making the roundtrips, via the process called inducedemission to start the lasing action. Accordingly, the energy in thecavity decreases, as shown in FIG. 5A, whereas the energy in the lasingpulse builds up in a gain process, as shown in FIG. 5B. In FIG. 5B thepeaks in the t(gain) interval represent the energy of the lasing pulseas it passes a particular point in the cavity, whereas the solid risingcurve is an envelope representing the energy gain averaged over asliding roundtrip period.

It is noted that implementations which trap a single incoming pulse inthe cavity can transfer just about all of the energy stored in the lasercrystal 310 into the single lasing pulse during its roundtrips. Incontrast, some implementations may allow multiple pulses into thecavity. However, in these examples the resulting laser beam may have alower energy per pulse thus reducing the energy per pulse below levelswhich are customary and beneficial for the relevant type ofphotodisruption.

After the energy pumped into the cavity is transferred to the lasingpulse with a high efficiency during a sufficient number of roundtrips,the controller-driver electronics may stop applying the high voltage tothe switchable polarizer 330, causing it to resume rotating thepolarization plane of the lasing pulse. Because of the restart of thepolarization rotation, the amplified laser pulse is then reflected outfrom the cavity by the thin film polarizer 340 at the end of the nextroundtrip, at a time denoted t(dump).

The dumping of the amplified laser pulses can be controlled in differentways. In some cases design calculations and computer methods can berelied upon to set the number of roundtrips after which the dumping isperformed. In others, prior calibration can be used to set the number ofroundtrips. In yet other cases, a wide variety of sensors can be coupledinto the optical path of the resonant cavity. This sensor or sensors cansense when the energy of the amplified lasing pulses reaches apredetermined value and send a control signal to a controller to dumpthe cavity accordingly.

Reflecting the amplified laser pulse out from the cavity andtransmitting it towards the compressor 400 completes the pump-gain-dumpcycle. Once the pulse-dumping is complete, the cavity returns to its lowQ state, restarting the pump-gain-dump cycle anew. In some designs, thepulse-injection port and the pulse-dumping port may be different. InFIG. 4 both of these ports are implemented in the thin film polarizer340.

In some implementations the lasing pulses perform 50-500 roundtrips, inother examples 100-200 roundtrips inside the cavity to enable thetransfer of energy from the pumped state of the laser crystal 310 to thelasing pulse. As discussed before, the oscillator 100 can create a seedpulse train with a frequency in the range of 10-200 MHz, in some casesin the range of 20-50 MHz. In some implementations, the laser engine 1or l′ outputs a laser pulse train with a repetition rate in the rangesof 10 kHz-2 MHz, or 50 kHz-1 MHz, or 100 kHz-500 kHz. Thus, theswitchable polarizer 330 decimates the incoming seed pulse train bytrapping only every 5^(th)-20,000^(th) seed pulse for amplification. Thetiming of these trapping sequences can be controlled by using theoscillator 100 as a master-clock.

The repetition rate is a central characteristic of a laser engine. Alarger variety of functionalities can be achieved if (1) the repetitionrate can be varied in a range of frequencies, and (2) the top of therange is high. For example, a cataract procedure may be optimallyperformed at a first repetition rate while a second repetition rate maybe better for a corneal procedure. A single laser engine can be used forboth of these functionalities if the laser engine can be adjusted tooperate both at the first and at the second repetition rate. Therefore,various design considerations will be reviewed next which can make therepetition rate variable and the upper limit of the range high in thelaser engines 1 and 1′.

As described in relation to FIGS. 3B-C and FIG. 4, the use of adispersion controller in the amplifier 300, such as a chirped mirror forany one of the mirrors 321-324, may compensate the dispersion of thelasing pulse caused by optical elements of the amplifier during aroundtrip in the cavity. This design feature allows the changing of therepetition rate of the laser engine 1 or 1′ without changing thecalibration, alignment or setup of the optical elements of the stretcher200 and compressor 200/400, such as the gratings 201, 203, 205, and 207,the lens 202 and the mirrors 204 and 208.

Instead of modifying the optical setup, the repetition rate change canbe achieved by applying electric control signals to modify the timingand operation of the laser engine 1. For example, the repetition ratecan be increased by applying control signals to reduce the repetitiontime t(rep)=t(recharge/pump)+t(gain).

Typically, the reduction of t(rep) is achieved by reducing both t(pump)and t(gain). The pumping time t(pump) can be shortened e.g. byincreasing the pumping intensity of the pumping diodes/lasers. The gaintime t(gain) can be shortened e.g. by reducing the number of roundtripsof the lasing pulse.

The energy of the laser pulse can be preserved in spite of the fewerroundtrips e.g. by increasing the energy gain per roundtrip. FIG. 5Billustrates the increase of the energy of the lasing pulse during thegain period as it passes a selected reference point in the cavityroundtrip by roundtrip. The ratio of the energies in subsequent passesis often characterized by the (“small signal”) gain factor g. The gainfactor g is sensitive to the total energy stored in the excited orpumped level of the laser crystal 310. The more energy stored, thehigher the g factor. Therefore, applying control signals to increase theenergy stored in the pumped level of the gain medium 310 can make thelasing pulse reach the same energy in fewer roundtrips, thus increasingthe repetition rate.

The upper limit of the repetition rate range can be increased in avariety of ways as well. In embodiments with a larger gain factor gfewer roundtrips are needed to achieve the same amplification. Thus,some implementations achieve a high upper limit of the repetition rateby employing a laser crystal 310 which has a higher gain factor g.

Also, since the gain factor g is sensitive to the total energy stored inthe excited or pumped level of the laser crystal 310, pumping theexcited level with more energy is another way to achieve a shortert(gain) and thus a higher repetition rate.

Another factor controlling the repetition rate is the time one roundtriprequires. The lasing pulse passes by a reference point at time intervals2 L/c where L is the length of the optical pathway in the cavity and cis the speed of light. Thus, in some embodiments the length L of theoptical pathway can be reduced to reduce the time of a roundtrip. Inthese implementations the same number of roundtrips and thus thetransfer of the same amount of energy takes a shorter time t(gain),increasing the repetition rate in yet another way.

Implementing one or more of the above discussed design principles,embodiments of the laser engine 1 or 1′ can operate with a repetitionrate up to 500 kHz, 1 MHz, or in some cases 2 MHz.

Additionally, in these implementations the reduction of t(gain) allowsthe use of a larger portion of the total repetition time t(rep) forsupporting a more favorable duty for the pump and dump cycle.

An often-used definition of the duty is the length of the low Q perioddivided by the length of the total period. Using this definition, in animplementation with e.g. a 400 kHz repetition rate, reducing t(gain)from 1 μsec to 0.5 μsec increases the duty from 0.6 to 0.75, a sizeableincrease of 25%.

Returning to the design principle of shortening the length L of theoptical pathway, it is noted that L is controlled, among others, by howfast the switchable polarizer 330 can switch to trap a pulse in thecavity. In a 1 meter optical pathway cavity the time of a roundtrip is 2L/c=6.6 ns. Accounting for the finite spatial extent of the pulse aswell, single pulse implementations therefore have a switchable polarizer330 with a switching time below 5 ns, others below 4 ns, or even below 3ns.

In some amplifiers the switchable polarizer 330 can be a Pockels cell.Pockels cells often apply a strong electric field to rotate thepolarization of incident light beams. The rotation of the polarizationis proportional to the first power of the electric field and thus can bequite strong. The Pockels effect occurs in crystals that lack inversionsymmetry, such as lithium niobate or gallium arsenide and othernoncentro-symmetric materials.

By sometimes applying kilovolts of voltage, Pockels cells can beswitched from a polarization-rotating state to apolarization-non-rotating state with a very short rise time. One measureof the rise time is the “5-95 time”, the time it takes for the rotationof the polarization plane to rise from 5% of the maximum/saturationvalue to 95% of it. In some implementations the rise time can be lessthan 5 ns, in others less than 4 ns, in yet others, less than 3 ns. Infact, in some implementations, the rise time is limited not by thedynamics of the Pockels cell itself, but rather by that of the switchingelectronics. Some implementations may use an innovative control anddriver circuit to enable this fast power switching process.

As described above, the shortening of the switching time of the Pockelscell is an effective way to shorten t(gain), enabling a fasterrepetition rate. Furthermore, these faster Pockels cells also allow thereduction of the length of the optical pathway and thus the size of thecavity.

Further, implementations of the laser engine 1 can be made to have feweroptical elements than some existing lasers. This is due in part to theapplication of the dispersion controller or compensator, obviating theneed for adjustable optical elements in the compressor, as well as tothe integrated stretcher-compressor architecture 200.

While some lasers may contain hundred or more optical elements, in someimplementations of the laser engine 1 the number of optical elements maybe less than 75. In others, less than 50.

In some implementations the number of optical elements in portions otherthan the oscillator can be less than 50. In others, less than 35.

Here the term “optical element” refers to any element which impacts anoptical property of a light beam. Examples include: a mirror, a lens, aparallel plate, a polarizer, an isolator, any switchable opticalelement, a refractive element, a transmissive element, and a reflectiveelement.

Optical elements are defined by surfaces where the light enters from theair and exits into the air. Therefore, a functional block, such as anobjective, is not one “optical element” if it contains several lenses,even if the lenses rigidly move together when the objective is moving.This is so because between the lenses of the objective the light doespropagate in air, however short is the separation. Even if two lensestouch each other without an airgap at their center, off-center beamsstill exit one lens into the air before entering the other one, and thusare counted as two optical elements. It is noted that schematicdescriptions of lasers often show fewer optical elements than what isnecessary for the actual functioning of the laser. Typically, when asingle lens is shown, its functionalities cannot be performed by anactual single lens, only by a carefully designed lens-assembly. Thus,such schematic descriptions are typically meant to be illustrative onlyand would be inoperable if implemented literally.

Implementations of the laser engine 1 with fast Pockels cells, fastswitching electronics and a small number of optical elements can have anoptical pathway inside the cavity shorter than 2 meters, others shorterthan 1 meter. Correspondingly, the total optical pathway of the laserengine from the generation of the photons in the oscillator 100 andincluding all the roundtrips inside the cavity of the amplifier 300 canbe less than 500 meters, or 300 meters, or even 150 meters.

Existing femtosecond lasers have a total optical pathway of 500 metersor longer and a cavity end-mirror-to end-mirror distance of 3-4 metersor longer because it is prohibitively difficult to shorten the opticalpathway below these values without the here-described innovativesolutions.

The list of innovative subsystems and features which can contribute tothe reduction of the size of laser engine 1 includes: (i) a fiber-basedoscillator 100 instead of a free-space oscillator; (ii) an integratedstretcher-compressor 200, possibly based on a single Chirped VolumeBragg Grating, which does not have optical elements to be adjusted whenthe repetition rate is changed; (iii) a dispersion-compensated amplifier300, eliminating the need for adjustable optical elements in thestretcher-compressor 200 when changing the repetition rate; (iv) anunusually fast-switching Pockels cell; (v) an unusually fast controlelectronics which can operate with fast rise times at the high voltagesof the Pockels cell including the kilovolt range; and (vi) a smallnumber of optical elements, requiring less space for accommodation.

Laser engines which implement a combination or all of these features cansupport an overall free-space optical path length of less than 500meters, in some implementations less than 300 meters and in some lessthan 150 meters.

Also, the amplifier 300 with some or all of the above relevant featurescan have an end-mirror-to-end-mirror optical pathway length of less than2 meters, in some cases less than 1 meter.

In many implementations the optical pathway is multiply folded, thus thephysical extent of the resonant cavity can be considerably shorter thanthe length of the pathway. Short and folded optical pathways cantranslate into a small overall extent of the amplifier 300. In somecases, none of the edge sizes of the amplifier 300 exceeds 1 meter, inother cases, 0.5 meter.

Correspondingly, the footprint of the entire laser engine 1, i.e. thearea it covers on the deck of a laser system, may be less than 1 m², inothers 0.5 m², in yet others 0.25 m², and possibly less than 0.1 m².Each of these areas or footprints can lead to distinctly new advantages.

The amplifier 300 and the laser engine 1 can have this unusually smallspatial extent because of using one or more of the above describeddesign principles and components. As such, the spatial extent canlegitimately distinguish the amplifier 300 and the laser engine 1 fromother lasers which do not employ these design principles and components.

Another consideration also deserves mention: it is critically simpler toservice subsystems which are on the top deck of a laser system and arethus accessible by simply removing a cover but without moving systemblocks in and out from the chassis of the laser system. Doing so canendanger the sensitive alignments of the system blocks in a customerenvironment (such as a hospital), where precision equipment is typicallynot available to restore the alignment. Thus, while stacking the variouscomponents of a surgical laser system, on top of each other may seem asanother way to reduce its footprint, doing so would introduceprohibitive challenges for the service of the laser system.

Therefore, reducing the size of the laser engine 1 allows the placementof other subsystems on the top deck of the laser system which alsorequire access for maintenance. Such additional subsystems may introducequalitatively new functionalities, thus critically improving the utilityof the overall laser system. Such additional subsystems can include animaging system to guide an ophthalmic surgery.

To summarize, the above features, alone or in combination, can beimplemented to construct physically compact laser systems. Such a smallspatial extent can be a valuable asset for at least the followingreasons: (i) ophthalmic surgical laser systems are often deployed invery crowded operating theatres where space and access is at a highpremium, favoring laser systems with small footprints; (ii) theserviceability of the laser engine is qualitatively better if most orall of its optical components fit on the top deck of the chassis of thelaser system; and (iii) small laser engines allow the deployment ofadditional systems on the top deck, adding critical new functionalitiesto the overall laser system, such as imaging systems to guide theophthalmic surgery.

Returning to tracking the path of the amplified stretched laser pulses,FIG. 2 illustrates that, once emitted by the amplifier 300, theamplified pulse can be forwarded back to the Faraday isolator 500. Oneof the functions of the Faraday isolator 500 can be to redirect theamplified pulses away from the oscillator with near-100% efficiency,thus preventing damage to the oscillator 100 by the amplified pulse.

In some cases the amplified pulses are directed to a compressor port ofthe stretcher-compressor 200 via polarizers 550 and 560. As describedabove, the stretcher-compressor 200 can re-compress the amplified pulsesand emit a pulsed laser beam with femtosecond pulses.

Implementations of the laser engine 1 utilizing the various solutionsdescribed above can output a laser beam with pulse duration in the rangeof 1-1000 femtoseconds (fs), in some cases 50-500 fs, in yet others100-300 fs. These femtosecond pulses can reach unusually high energies,e.g. energies in the range of 1-100 μJoule/pulse, in others 10-50μJoule/pulse, in yet others 20-30 μJoule/pulse.

These pulse energies can enable useful applications which are notaccessible for lasers whose pulse energy is less than 1, 10 or 20μJoule/pulse, because there are several different laser-tissueinteractions in the eye which exhibit a threshold behavior. There aresurgical procedures where laser pulses below 1 μJoule/pulse energies donot cause the surgically desired tissue modification. In other surgicalprocedures this threshold can be 10, or 20 μJoule/pulse.

For example, cataract surgery requires directing the laser deep in thetarget tissue, such as to a depth of 10 mm. This requirement constrainsthe numerical aperture, thus calling out for higher energy per pulsevalues to produce photodisruption. In some cases 10-15 μJoule/pulseenergies can be sufficient. To avoid operating at the maximum energyvalues, devices with 20 μJoule/pulse can be desirable. As these numbersare on-target energies, to account for losses along the optical path,the laser system may include lasers which output 25-30 μJoule/pulse.

For example, in a cataract surgical application, cutting cataracts ofhardness 1, 2, 3, or 4 may necessitate laser pulse energies abovecorresponding thresholds. For example, under certain circumstanceslasers with pulse energies higher than 10-15 μJoule/pulse can cutcataracts of hardness 1, pulse energies higher than 10-20 μJoule/pulsecan cut cataracts of hardness 2, pulse energies higher than 20μJoule/pulse can cut cataracts of hardness 3 and pulse energies higherthan 30-50 μJoule/pulse can cut cataracts of hardness 4. These thresholdenergies can be impacted by several factors, including the pulse length,the repetition rate, the location of the laser spot within the overalltarget region, and the age of the patient.

The effect of the laser pulses is a highly non-linear function of itsparameters in wide classes of target tissues. Therefore, lasers with thesame energy/pulse but different pulse duration may reach differentresults in the surgical targets. For example, picosecond pulses with aspecific energy/pulse value may generate bubbles in ophthalmic tissuewhich expand uncontrollably, whereas femtosecond pulses with a similarenergy/pulse may create bubbles which remain controlled. Accordingly,the above described energy/pulse values can be generated by laserengines emitting femtosecond pulses, i.e. pulses with a length of lessthan a picosecond.

The strength of the laser beam can be quantified in terms of its poweras well. E.g. a 20 μJoule/pulse laser with a 50 kHz repetition ratecarries 1 W power. Expressed in terms of power, the above describedthreshold values can translate to threshold powers of 0.1 W, 1 W, and 10W at corresponding repetition rates. Thus, laser engines capable ofemitting laser beams with a power in excess of these thresholds offerdifferent functionalities.

For example, the Food and Drug Administration classifies medical lasersby their power. The laser class 3B is often used for ophthalmicprocedures as its effects have been widely studied. Lasers which outputbeams with a power less than 0.5 W of power belong to the class 3B.Therefore, lasers with a power less than 0.5 W offer substantiallydifferent applications than lasers with a higher power.

FIGS. 6A-D illustrate a functionality of the laser engine 1, takingadvantage of its capability of changing the repetition rate at a highspeed. In various applications the surgical laser beam causesphotodisruption at a focus point, wherein the disrupted regioneventually expands into a bubble. As the focal spot is scanned by ascanning optics of the laser system at a scanning speed, a string ofbubbles gets generated. These strings of bubbles can form lines orsurfaces in a controllable manner. The large number of bubbles reducesthe mechanical integrity of the target tissue along these lines orsurfaces, making it possible to easily separate the target tissue alongthe lines or surfaces. In effect, the scanned laser beam “cuts” thetarget tissue along these lines or surfaces.

In some representative cases the bubbles may be a few microns (μ) indiameter, separated by distances of the order of 10-50μ, or more. Thesurgical laser system typically creates a bubble once every repetitiontime, the inverse of the repetition rate. Therefore, the bubbles areessentially equally spaced as long as the scanning speed of the lasersystem is constant.

Bubbles expand after they have been created by the laser pulse. Undervarious circumstances this expansion can become uncontrolled. Such anuncontrolled bubble expansion can strongly scatter the subsequent laserpulses in the target region, seriously undermining the precision andcontrol of the ophthalmic surgery. Forming the bubbles too close to eachother is one of the triggers of such an uncontrolled expansion, as itcan cause the bubbles to coalesce. Other possible processes involve theexpansion of a bubble can interfere with the formation of thesubsequently formed bubbles, causing a cross-talk between them, onceagain leading to the uncontrolled expansion of the bubbles. Therefore,maintaining a predetermined bubble separation during scanning can be ahigh priority to retain control over the bubble expansion for ophthalmicsurgical laser systems.

However, the scanning of the focal spot typically involves moving partssuch as mirrors and galvos. Given the extremely short repetition times,even the smallest inertia and mechanical delay of these moving parts canimpact the bubble densities. For example, when scanning along somesurgical patterns, the scanning speed may slow down at turning pointsand corners, possibly leading to an increased density of laser spots andthe bubbles. In other cases, simply the geometry of the surgical patternleads to an enhanced areal density of the bubbles even if the linearbubble density is kept constant.

FIG. 6A shows the example when a fixed repetition rate laser is scanningthrough a switchback surgical scanning pattern in order to create aseparation sheet in the target tissue. However, approaching theturnaround or switchback points, the scanner slows down while therepetition rate remains constant and thus creates an increased linearand thus aerial bubble density, as shown. Such an increased bubbledensity can lead to serious control problems, as described above.

This technical issue is addressed in some existing laser systems byincluding additional elements, such as a beam blocker, which interruptsthe laser beam upon approaching such turning points to prevent theformation of high bubble-density regions. However, including such beamblockers means adding additional elements in the laser system, whoseoperation is to be controlled and synchronized with the scanning itself.All of these additions mean further challenges and increased complexity.

Similar problems arise even when the scanning simply comes to the end ofa line in a scanning pattern, again slowing down of the scanning speedand causing an increased linear bubble density.

FIG. 6B shows that such sharp turnaround points can be avoided byfollowing “acceleration-minimizing” scanning patterns. An example of anacceleration-minimizing pattern is a spiral, which has no sharpswitchbacks. However, even a spiral pattern only decreases theacceleration but does not eliminate it. Therefore, the scanning speedstill varies in these systems and thus the fixed repetition rate stillhas to be selected so that the bubble density does not increase above athreshold value even at the lowest speed sections of the pattern. Thisdesign principle, however, means that for most of the pattern thescanning speed is lower than the system could support in order toachieve the bubble density necessary to achieve the cutting orseparating function. Equivalently, if a higher scanning speed isutilized then the separation of the bubbles may get smaller, leading toan interference or cross-talk between the forming bubbles. All of theseeffects increase the danger of uncontrollable or non-deterministicbubble expansion.

Implementations of the laser engine 1 can be designed to offer a usefulfunctionality in this context. The unique design in general and thedispersion controller of the amplifier 300 in particular makes itpossible to change the repetition rate essentially synchronously withthe changing scanning speed. In some laser engines the repetition ratecan be changed in a change-time within the range of 10 μs-1 s, in somespecial cases in the range of 1 μs-1 s. Therefore, some implementationscan include control electronics to slow down the repetition rate of thelaser engine 1 according to a designed or measured slowdown of thescanning speed along the surgical pattern to maintain a near constantbubble density in the target region. Such approximately constant bubbledensity can be achieved, for example, by changing the repetition rateproportionally with the varying scanning speed. With this functionalitythe laser engines 1 or 1′ may be able to form bubbles with a near evenlinear or areal bubble density or separation and thus prevent orcounteract an uncontrolled bubble expansion.

FIG. 6C illustrates a scanning surgical pattern with the sameswitchbacks as in FIG. 6A, where the repetition rate is reduced as thescan moves around the switchback, generating a cut with an essentiallyeven linear separation between the bubbles.

FIG. 6D illustrates a spiral surgical pattern with a reduced repetitionrate as the spiral converges to the center, where bubbles would havebeen too close to each other without this reduction. This embodiment istherefore once again capable of creating an essentially even arealbubble density.

Of course, the rapid variability of the repetition rate also allows thecreation of bubbles not only with a constant density, but with apredetermined density profile as well. For example, the nucleus of theeye is harder towards its center. Therefore, in some implementations,the bubble density may be increased as the scan crosses the center ofthe nucleus, followed by a decrease past the center. A large number ofdifferent density profiles can have different medical advantages andbenefits. The density profile can be also adjusted not on apredetermined basis but in response to an imaging or sensing of thetarget region.

FIGS. 7A-D illustrate yet another design feature helping laser enginesto change the repetition rate essentially synchronously with thescanning, or at least within the times scales of the ophthalmic surgery,e.g. within 60-120 seconds.

FIGS. 7A-B illustrate the phenomenon called thermal lensing and itsimpact on laser design. When the laser crystal 310 is pumped by the pumpdiodes and then transfers its energy by amplifying the laser pulse, itstemperature T rises. The temperature T often rises unevenly: typicallythe temperature is highest in the pumped center region, possibly peakingat or around the optical axis, and decreases with increasing radialdistance.

There are at least two effects of this uneven temperature rise: (i)since the index of refraction n increases with the temperature: n=n(T),it exhibits a maximum in the center region of the laser crystal 310; and(ii) the increasing temperature makes the center region of the lasercrystal 310 thermally expand more extensively than its surroundingregion and therefore bulge, held by the colder outer region. Both ofthese effects tend to focus the incident parallel rays. This phenomenonis called thermal lensing. This thermal lensing is referred to bysymbolizing the laser crystal with a lens 310′. The thermal lens canexhibit refraction by several diopters and thus it can alter theperformance of the laser engine substantially.

FIG. 7A illustrates that the design of a laser engine typically involvesdetermining the refractive effects of the thermal lensing by the lasercrystal at the operating temperature T=Top, determined by the operatingrepetition rate and beam power, and introducing refractive compensationfor the thermal lensing via other optical elements of the laser engine.An example is to introduce an additional lens 312, which can restore theconvergent beam to a parallel beam after it was focused by the thermallens 310′.

FIG. 7B illustrates that such a refractive compensation is appropriatefor a particular operating temperature T=Top and thus for a particularrepetition rate and beam power only. Indeed, if an application calls fora change of the repetition rate or power, the changed repetition rateand/or the changed power changes the temperature T of the laser crystal310 from T=Top to T=Top′. This change in temperature changes thefocusing by the thermal lens with it (from the convergent beamrepresented by the dotted lines to the one with solid lines),transforming the beam which was parallel at T=Top to diverge at T=Top′,thus having poorer convergence properties.

FIG. 7B also illustrates that the convergence properties can be restoredby adjusting the refractive compensation. Changing the refractivecompensation typically requires adjusting one or more optical element ofthe laser engine, such as moving a lens, rotating a grating, or movingthe beam relative to the optical axis. FIG. 7B shows an adjustment ofthe compensating lens 312 along the optical axis, as indicated by thearrow. Analogously to the previously dispersion compensation, thisrefractive compensation via mechanical adjustments is also slow andrequires fine tuning and calibration. Therefore, most lasers sidestepthis challenge entirely and do not allow for the changing of therepetition rate. And even in those lasers which offer a changeablerepetition rate, the rate cannot be changed near synchronously with thescanning of the laser engines, not even within ophthalmic surgical timesbecause the slowness of the adjustment of the compensating opticalelements.

FIGS. 7C-D illustrate implementations of the laser engine 1 employingvarious design principles to minimize the effect of thermal lensing. Therefraction by the thermal lens 310′ can be reduced by a considerabledegree if most or all the rays propagate through or very close to thecenter of the thermal lens 310′, because rays crossing a lens at itscenter do not get refracted on the level of the geometrical opticsapproximation. On the level of wave optics and when including the finiteextent of the lens, these central rays do get refracted, but only to aminimal degree.

FIG. 7C illustrates that the rays can be compressed to hit the center ofthe lens e.g. by (i) using an embodiment of the end-mirror 322 which hasa focusing effect; (ii) placing the thermal lensing laser crystal310/310′ very close to the focal point of the focusing end-mirror 322 sothat most of the rays from the focusing end-mirror 322 hit the center ofthe thermal lensing laser crystal 310/310′; and (iii) placing the otherend-mirror 321 also very close to the focal point of the focusingend-mirror 322 and thus to the lensing crystal 310 to ensure that thebeam reflects back into itself instead of becoming divergent. In suchdesigns, when the repetition rate, or the power of the beam is changed,thus changing the temperature of the laser crystal 310 from T=Top toT=Top′, there is no pressing need to readjust any mechanical or opticalelement of the laser engine 1, since the refractive impact of the lasercrystal 310 has been minimized. Thus, the repetition rate, or the powerof the beam, can be changed without any corresponding adjustment of arefractive compensator.

Referring to FIG. 4, in various embodiments any one or more of theend-mirrors and folding-mirrors 321-324 can have the described focusingeffect.

The designs parameters of this embodiment, including d1, the distance ofthe end-mirror 321 and the lensing crystal 310, d2, the distance of thelensing crystal 310 and the focusing end-mirror 322, and otherparameters, such as apertures, thickness of the lensing crystal 310, andradii of the focusing end-mirror 322, can be optimized to furtherminimize the already reduced thermal lensing.

FIG. 7D illustrates a related design. In this embodiment bothend-mirrors 321 and 322 are of the focusing type. This example furtherreduces the thermal lensing as the laser crystal 310 can be placed intothe shared focal point of the two end-mirrors with higher precision.Again, the other parameters can be made subject to an additional designoptimization.

FIG. 8 illustrates a quantitative characterization of the suppression ofthermal lensing in the laser engine 1. The horizontal axis shows theratio of the operating temperature of the center of the crystalToperating=Top to the ambient temperature Tambient. The vertical axisshows the optical power of the laser beam emitted by the laser engine 1.The graph shows that, even if the lasing operation heats up the laserengine 10—50% above the ambient temperature, the optical power variesonly by a few %, reaching about 10% at Toperating/Tambient=150%. Theoptical power of the laser crystal 310 changes so little over such awide range of operating temperatures because the refractive impact ofthe thermal lensing of the laser crystal 310 is minimized efficiently bythe designs of FIG. 7C and FIG. 7D.

The above detailed description provides design principles and examplesthat can be used to achieve a functionality of changing the repetitionrate without the need of making adjustments of optical elements outsidethe oscillator 100, including (i) using dispersion compensation insidethe amplifier 300; (ii) using an integrated stretcher-compressor 200;and (iii) using cavity architectures which minimize thermal lensing, aswell as other design considerations described above. Laser engines usingone or more of the above design features or analogues can enable thechanging of the repetition rate in repetition rate ranges withinchanging times, causing only limited laser beam modification.

Here the repetition rate range can be 10 kHz-2 MHz, or 50 kHz-1 MHz, or100 kHz-500 kHz, each of these ranges offering specific functionalities.

The changing time can be the time scale of a multi-step ophthalmicsurgery, such as within the range of 1-120 seconds, or 10-60 seconds or20-50 seconds, depending on the type of surgery. Laser engines with achanging time in these ranges can support a change of the repetitionrate to switch from a rate necessary for a first surgical procedure to arate necessary for a second surgical procedure.

In other cases, such as in the embodiments described in relation toFIGS. 6A-D, the changing time can be a time scale set by the scanningspeed of the laser system, e.g. a multiple of the repetition times,where the multiple can be in the range of 1-10,000, or 100-1,000. Sincethe repetition time is about 100 microseconds (100 μs) at 10 kHz and 1μs at 1 MHz, these “scanning-changing times”, or “scanning-synchronouschanging times” can be in the range of 1 μs-1 s.

A linear density of the bubbles is preserved in some implementations bychanging the repetition rate in response to the change of a scanningspeed so that a ratio of the scanning speed and the repetition rateremains essentially constant.

The laser beam may get modified to a limited degree by the repetitionrate change. This modification can be captured in various ways,including: (i) the beam diameter changes by less than 10% or 20%; or(ii) the center of the beam moves by less than 20% or 40% of the beamdiameter. Here the beam diameter can be defined in different ways, suchas the diameter where the intensity of the beam falls to 50% of theintensity at the center of the beam. Other definitions can be used aswell.

An example is a laser engine 1 which can emit a laser beam with arepetition rate of 100 kHz and beam diameter at the focal spot of 3microns, where the repetition rate of the laser beam can be changed to150 kHz by adjusting only the oscillator 100 in a changing time of 15seconds, and in spite of this considerable change, the beam is modifiedonly to a limited degree: the focal spot diameter changes by only 15% to3.45 microns and its center moves relative to the optical axis only by30% of the beam diameter, i.e. by 0.9 microns. Such a laser engine canbe used to perform a cataract surgery with the 100 kHz repetition rate,have its repetition rate changed to 150 kHz in 15 seconds and be usedagain to perform a subsequent corneal procedure with the 150 kHzrepetition rate, the entire procedure taking no more than 100 or 120seconds, while maintaining a very good beam quality.

In another example the laser engine 1 can emit a laser beam with arepetition rate of 100 kHz and beam diameter of 4 microns. When thescanning is approaching a sharp switchback of a surgical pattern wherethe scanning speed slows down to half of the regular scanning speed, therepetition rate can be accordingly slowed gradually to half of itsvalue, i.e. from 100 kHz to 50 kHz to maintain a near-constant lineardensity of the generated bubbles or spots. If this slowdown is performede.g. in 10 repetition times of the 100 kHz repetition rate, then thetotal time of changing the repetition rate is about 100 μs.

The repetition rate can be changed in several steps or gradually, thenet result being that the repetition rate is changed near synchronouslywith the changing of the scanning time scale of the laser beam, from 100kHz to 50 kHz in about 100 μs. The design of the laser engine 1 makes itpossible to change the repetition rate in this remarkably fast timewhile maintaining a high laser beam quality. In an example, the laserbeam diameter can be 4 microns at 100 kHz, which changes only by 10% to3.6 microns as the repetition rate decreases to 50 kHz, and the centerof the laser beam moves away from the optical axis only by 20% of thebeam diameter, i.e. by 0.8 microns.

Yet another way to express how the laser engine 1 is capable ofmaintaining the high beam quality while changing the repetition rate isin terms of the well-known g1-g2 stability plane. Implementations of thelaser engine 1 can keep the beam parameters g1 and g2 within thehyperbolic stability region in a wide range of repetition rates, e.g. inthe 10 kHz-2 MHz, or 10 kHz-500 kHz, or 50 kHz-200 kHz range.

The small number of optical elements can be a critical anddistinguishing characteristic of implementations of the laser engine 1from yet another vantage point. Femtosecond lasers in general arecutting edge devices, very sensitive to and easily misaligned byenvironmental impacts, usage different from the instructions, and evenstraightforward wear, such as self-heating effects. Therefore, theoptical elements of femtosecond lasers can require fine tuning,readjustment and maintenance in regular short time intervals. Typicalfemtosecond lasers may contain hundred or more optical elements and themalfunction of any one of those optical elements can cause themalfunction of the entire laser.

Some typical lasers can malfunction as often as after 30-60 “cycling”,i.e. switching a power of the laser engine on and off. To preemptmalfunctions happening in operation, operators of some laser systemshave to plan regular and costly maintenance visits, with all theattendant costs and down-times, and can still run a high risk of in-situmalfunction with disruptive consequences.

In contrast, the embodiments of the laser engine 1 can be cycled morethan 120 times by switching a power on and off without needing toreadjust any optical element of the laser engine 1. For some embodimentsthe number of cycles can be more than 180 or even 240.

In surgical operations, to minimize problems associated with the heatingand cooling of the laser crystal 310, often the laser is switched ononce in the morning and switched off only in the evening, i.e. surgicallasers are often cycled once a day. In a simple estimate, if lasers areused five times a week, thus approximately 20 times a month, then 30cycling can translate to a high chance of malfunction after 1.5 month,and 60 cycling to 3 months.

In contrast, some implementations of the laser engine 1 can be cycledmore than 120 times, translating to 6 months of low probability ofmalfunction. Other implementations can be cycled 180 or 240 times,translating into 9 months or a full year of low probability ofmalfunctions. Therefore, embodiments of the laser engine 1 can beoperated by a preventive maintenance schedule which poses significantlylower burden on user and service provider alike. Also, such a lowfrequency maintenance schedule makes possible different types ofmaintenance, such as replacement of entire sections of the laser system.In some cases the entire laser engine 1 can be simply replaced by afreshly maintained one on-site and the maintenance of the laser engine 1can take place in the high tech environment of a service provider'sbase, instead of the lower tech environment of a surgical operator.

While this document contains many specifics, these should not beconstrued as limitations on the scope of an invention or of what may beclaimed, but rather as descriptions of features specific to particularembodiments of the invention. Certain features that are described inthis document in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or a variation of a subcombination.

A number of implementations of imaging-guided laser surgical techniques,apparatus and systems are disclosed. However, variations andenhancements of the described implementations, and other implementationscan be made based on what is described.

What is claimed is:
 1. A variable repetition rate laser engine,comprising: an oscillator that generates and outputs a beam offemtosecond seed pulses; a stretcher-compressor that stretches aduration of the seed pulses; and an amplifier that receives thestretched seed pulses from the stretcher-compressor, amplifies anamplitude of selected stretched seed pulses to create amplifiedstretched pulses, and outputs a laser beam of amplified stretchedpulses; wherein the stretcher-compressor receives the laser beam ofamplified stretched pulses, compresses a duration of the amplifiedstretched pulses, and outputs a laser beam of femtosecond pulses with apulse duration of less than 1,000 femtoseconds; and the amplifiercomprises a dispersion compensator that reduces a dispersion of theamplified stretched pulses, caused by optical elements of the amplifierand an electro-optical switchable polarizer, configured to operate at arepetition rate in the range of 50 kHz-1 MHz; wherein the laser engineoutputs a laser beam with an output power greater than 0.1 W.
 2. Thelaser engine of claim 1, wherein: the oscillator is a diode pumped fiberoscillator.
 3. The laser engine of claim 1, wherein: the oscillatoroutputs transform-limited seed pulses.
 4. The laser engine of claim 1,wherein: the oscillator generates the beam with a seed-pulse-duration ofless than 1,000 femtoseconds.
 5. The laser engine of claim 1, wherein:the oscillator outputs the beam with a seed pulse repetition rate in therange of 10-100 MHz.
 6. The laser engine of claim 1, wherein: thestretcher-compressor comprises a chirped volume Bragg grating.
 7. Thelaser engine of claim 1, wherein: the stretcher-compressor comprises aphotothermal refractive glass.
 8. The laser engine of claim 1, wherein:the stretcher-compressor stretches a duration of the femtosecond seedpulses by a factor greater than
 10. 9. The laser engine of claim 1,wherein: the stretcher-compressor stretches a duration of thefemtosecond seed pulses to a stretched duration of 1,000-200,000femtoseconds.
 10. The laser engine of claim 1, wherein: the laser enginedoes not contain a tunable stretcher-compressor.
 11. The laser engine ofclaim 1, comprising: a polarizer and a λ/4 plate between the oscillatorand the stretcher-compressor that redirects the beam of stretched seedpulses toward the amplifier.
 12. The laser engine of claim 1,comprising: a Faraday isolator that receives the beam of stretched seedpulses from the stretcher-compressor, outputs the beam of stretched seedpulses toward the amplifier, receives the laser beam of amplifiedstretched pulses from the amplifier, outputs the laser beam of amplifiedstretched pulses towards a compressor port of the stretcher-compressor,and isolates the oscillator from the laser beam of amplified stretchedpulses.
 13. The laser engine of claim 1, wherein: the dispersioncompensator introduces a dispersion opposite in sign to a dispersionintroduced by an optical element of the amplifier.
 14. The laser engineof claim 13, wherein: the dispersion introduced by the dispersioncompensator is essentially equal in magnitude and opposite in sign to adispersion introduced within one roundtrip by the optical elements ofthe amplifier other than the dispersion compensator.
 15. The laserengine of claim 1, wherein the dispersion compensator comprises: atleast one of a chirped mirror, a chirped fiber, a chirped grating, aprism, or a chirped transmissive optical element.
 16. The laser engineof claim 1, the amplifier comprising: a gain material that amplifies theamplitude of the selected stretched seed pulses; two end-mirrors thatdefine a resonant cavity; and two folding mirrors that fold a resonantoptical pathway inside the amplifier, wherein at least one of the twoend-mirrors and the two folding mirrors is a chirped mirror.
 17. Thelaser engine of claim 16, wherein: the chirped mirror introduces anegative dispersion to the amplified stretched pulses.
 18. The laserengine of claim 1, wherein: the laser engine is configured to output thelaser beam with a first repetition rate, and subsequently with adifferent second repetition rate with essentially the same alignment ofall optical elements of the laser engine.
 19. The laser engine of claim18, wherein: the first repetition rate and the second repetition rangefall within the range of 50 kHz-1 MHz.
 20. The laser engine of claim 1,wherein: the amplifier is configured to have a number of roundtrips ofthe amplified stretched pulses in the amplifier changed when arepetition rate is changed while keeping an optical setup of theamplifier unchanged.
 21. The laser engine of claim 1, wherein: theamplifier has an end-mirror-to-end-mirror folded optical pathway of lessthan 1 meter.
 22. The laser engine of claim 1, wherein the amplifier isone of: a cavity dumped regenerative amplifier, a chirped pulseamplifier or a Q-switched amplifier.
 23. The laser engine of claim 1,wherein: the electro-optical switchable polarizer is positioned in anoptical pathway between end-mirrors and is configured to selectstretched pulses by switching between a polarization-adjusting state inwhich the switchable polarizer adjusts a polarization of the amplifiedstretched pulses; and a polarization-non-adjusting state in which theswitchable polarizer essentially does not adjust the polarization of theamplified stretched pulses.
 24. The laser engine of claim 23,comprising: a high voltage power-switch that controls the switchablepolarizer to switch from the polarization-non-adjusting state to thepolarization-adjusting state with a rise time of less than 5nanoseconds.
 25. The laser engine of claim 1, wherein: the laser enginechanges a first repetition rate of the laser beam of femtosecond pulsesto a second repetition rate within 1-120 seconds.
 26. The laser engineof claim 1, wherein: the laser engine changes a first repetition rate ofthe laser beam of femtosecond pulses to a second repetition rate withina changing time in the range of 1 μs-1 s.
 27. The laser engine of claim1, wherein: the femtosecond pulses of the laser beam have an energy inthe range of 1-100 μJ/pulse.
 28. The laser engine of claim 1, wherein:the laser engine is part of an ophthalmic surgical system.
 29. The laserengine of claim 1, wherein: the dispersion compensator is configured tocompensate a group delay dispersion in the range of 5,000-20,000 fs².30. The laser engine of claim 1, wherein: the variable repetition ratelaser engine is configured to vary the repetition rate, following arate-variation-protocol, by more than 10%.
 31. A method of generating alaser beam with a variable repetition rate laser engine, the methodcomprising the steps of: generating a beam of seed pulses with durationless than 1000 femtoseconds with an oscillator; stretching a duration ofthe seed pulses with a pulse stretcher; amplifying an amplitude ofselected stretched seed pulses with an amplifier to generate amplifiedstretched pulses; compressing a duration of the amplified stretchedpulses to below 1,000 femtoseconds with a pulse compressor; outputting alaser beam of femtosecond pulses with an electro-optical switchablepolarizer with a first repetition rate in the range of 50 kHz-1 MHz andwith a pulse duration less than 1,000 femtoseconds; changing therepetition rate from the first repetition rate to a second repetitionrate in the range of 50 kHz-1 MHz without essentially changing anoptical setup of the laser engine; and outputting the laser beam offemtosecond pulses with the second repetition rate and with a pulseduration less than a 1,000 femtoseconds, wherein the amplifyingcomprises utilizing a dispersion compensator in the amplifier to reducea dispersion of the amplified stretched pulses caused by an opticalcomponent of the amplifier; wherein the laser engine outputs a laserbeam with an output power greater than 0.1 W.
 32. The method of claim31, wherein the reducing the dispersion step comprises: introducing acompensating dispersion by at least one chirped mirror in the amplifier,wherein the compensating dispersion is essentially equal in magnitudeand opposite in sign to a dispersion introduced by all optical elementsof the amplifier other than the dispersion compensator per roundtrip.33. The method of claim 31, the changing the repetition rate stepcomprising: changing a number of roundtrips in the amplifier.
 34. Themethod of claim 31, wherein: the stretching step and the compressingstep are executed by the same stretcher-compressor.
 35. The method ofclaim 31, comprising: outputting the laser beam with the secondrepetition rate within 1-120 seconds after having finished theoutputting the laser beam with the first repetition rate.
 36. The methodof claim 31, comprising: changing the repetition rate from the firstrepetition rate to the second repetition rate in a changing time in therange of 1 μs-1 s.
 37. A variable repetition rate laser engine,comprising: an oscillator that generates a pulsed light beam with apulse duration of less than 1000 femtoseconds; a stretcher-compressorthat stretches the duration of the pulses of the light beam; and anamplifier that amplifies an amplitude of the stretched light pulses togenerate amplified stretched pulses and comprises a dispersioncompensator that at least partially compensates a dispersion introducedby optical elements of the amplifier, wherein the stretcher-compressorcompresses a duration of the amplified stretched pulses, and outputs abeam of laser pulses; and the laser engine is operable to output thebeam of laser pulses with an electro-optical switchable polarizer with afirst repetition rate in the 50 kHz-1 MHz range and subsequently with asecond repetition rate in the 50 kHz-1 MHz range, utilizing essentiallythe same alignment of all optical elements of the laser engine, aduration of the outputted laser pulses being less than 1000 femtosecondsfor the first and the second repetition rates; wherein the laser engineoutputs a laser beam with an output power greater than 0.1 W.
 38. Thelaser engine of claim 37, the amplifier comprising: a switchablepolarizer between end-mirrors of the amplifier that switches between astate in which the switchable polarizer adjusts a polarization of theamplified stretched pulses; and a state in which the switchablepolarizer does not adjust the polarization of the amplified stretchedpulses with a rise time of less than 5 nanoseconds.
 39. The laser engineof claim 37, wherein: the laser engine switches between the firstrepetition rate and the second repetition rate in a time less than 60seconds.